BACKGROUND Serum adenosine deaminase (ADA) activity increases in diseases where cellular immunity is involved. Since cell-mediated immune responses play a paramount role in the pathogenesis and healing of the visceral leishmaniasis, therefore, the present study was undertaken to evaluate the serum ADA activity in different pathological conditions. METHODS Adenosine deaminase was determined in sera of active visceral leishmaniasis (VL) patients at diagnosis and at posttreatment (n=22), healthy controls (n=15), patients with malaria (n=10), leprosy (n=10) and tuberculosis (n=10). RESULTS Serum levels of ADA were significantly higher in active VL patients as compared to controls and patients with other diseases. ADA levels were also raised in patients with malaria, though not significantly as compared with active VL patients. Sera from VL patients at posttreatment showed significantly decreased ADA levels over sera from patients at diagnosis. CONCLUSIONS The results therefore suggest that ADA is involved in the pathogenesis and could be used as a clinical marker in the diagnosis of visceral leishmaniasis.