UNLABELLED Acute tubulointerstitial nephritis (ATIN) is a rare renal disorder in children. Patients usually present non-specific symptoms and signs so that the diagnosis of ATIN is often delayed. The disease may be infection- or drug-induced or it may occur without a known cause. Early recognition and appropriated therapy usually lead to an excellent prognosis. The aim of the study was to describe clinical and laboratory findings and the course of ATIN in 21 patients, that are typical enough to enable early recognition of the disease as it is potentially reversible. METHODS Between 1986 and 1997 we observed 21 patients, aged 7-16 years (mean, 12.8), with acute tubulointerstitial nephritis, including eight with tubulointerstitial nephritis and uveitis (TINU syndrome). Laboratory studies included urinalysis, complete blood count, erytrocyte sedimentation rate (ESR), plasma creatinine, glomerular filtration rate (GFR), electrolytes, proteins, IgG, C3, C4 antinuclear-antibodies (ANA), antistreptolysin-O and antibodies to hantaviruses. Renal ultrasound was done in all patients. Renal biopsy was performed in 5 children. RESULTS All children had previously been healthy. The symptoms of ATIN developed within a few days (Table 1). The most common initial symptoms were fatigue, fever, gastrointestinal disturbances, anorexia and weight loss. Less common complaints included headache, arthralgias and maculopapular rash. On addmition no patient had hypertension, oedema or evidence of acute infection. ESR, plasma urea and creatinine concentrations were increased in all, plasma proteins and IgG levels in the majority of patients. ANA were negative in 15 pts in whom this analysis was performed; C3 and C4 levels were normal. In two children recent strepococcal and in the other 6 hantavirus infection was serologicaly proved. All patients had non-oliguric acute renal failure (ARF): GFR was 21.7 +/- 8 9 in 14 pts and 67 +/- 9.7 in 7 pts. Low urine specific gravity (1003-1014), mild proteinuria (0.3-0.4 g/24 h), leukocyturia and/or haematuria were found in all patients; glycosuria, aminoaciduria and decreased tubular reaposrption of phosphate (TRP) were found in 12/21, 9/21 and 9/14 patients, respectively. Urine cultures were negative in all children. Renal US demonstrated enlarged hyperechoic kidneys in 11 pts, in remaining 10 pts no abnormalities were found. Renal biopsy, performed in 5 children, confirmed the diagnosis of ATIN. Eight patients subsequently developed anterior uveitis as part of TINU syndrome. Treatment included supportive therapy in all and six patients received prednisolone for 4-8 weeks (40-60 mg/m2/24 h for 10-14 days with subsequent reduction of dose over several weeks). Anterior uveitis was successfully treated with topical steroids. Renal function completely recovered in all patients: GFR (109 +/- 22.6 ml/min) within a mean interval of 47 +/- 33 days, concentration ability within 2-12 (mean 4.5) months. DISCUSSION Common clinical features of ATIN are non-oliguric acute renal failure of various degrees, signs of tubular dysfunction, proteinuria, haematuria, leukocyturia and absence of hypertension. All our patients had normal blood pressure, non-oliguric renal failure, proteinuria, hypostenuria and abnormal urinary sediment; about half of them had glycosuria and/or other signs of proximal tubular dysfunction. The most important causes of ATIN in children reported in literature are systemic infections and drugs. However, the cause of ATIN in our patients was assessed as being related to infection only in 8 patients and to diclofenac in one. No infection, drug, toxin or other cause could be identified in 4, as well as in 8 pts with TINU syndrome. The prognosis of ATIN in children is considered to be favourable, but some patients may develop chronic renal failure. Renal function completely recovered in all our patients; that is consistent with outcome data from the most reports. CONCLUSION Acute tubulointerstitial nephritis is an important cause of ARF in children, its aetiology may be different and it carries an excellent prognosis. ATIN should be suspected in a child who presents typical, although non-specific symptoms and signs, associated with lukocyturia and/or microhaematuria, signs of tubular dysfunction and unexplained renal failure. The diagnosis can be verified at renal biopsy. Early recognition of the disease is important to remove possible aetiologic agents and to treat them before chronic lesions are present to avoid long-term renal damage.