A study was made of 20 patients fulfilling the criteria customarily used for the diagnosis of interstitial cystitis. A possible infective aetiology was sought by culture of bladder tissue, catheter and midstream specimens of urine, and urethral swabs by methods capable of detecting fastidious bacteria as well as aerobic pathogens. All bladder biopsies showed the histological appearances usually associated with interstitial cystitis, and bacteria were isolated from the catheter specimens and/or bladder biopsies of 12 patients. Eight of these isolates were fastidious bacteria, Gardnerella vaginalis (6) and Lactobacillus sp. (2). Fastidious bacteria were isolated from the midstream specimen of urine (MSU) and/or urethral swab of 6 other patients. The correlation of the histological and bacteriological findings supports the hypothesis of an infective aetiology and suggests that the so-called urethral syndrome and interstitial cystitis may be the earlier and later stages of the same disease process. The importance of early diagnosis of infection in these patients is emphasised.