This longitudinal study was conducted in BP Koirala Institute of Health Sciences (BPKIHS), a Medical University situated in eastern Nepal, between May 2001 and December 2001. The main objective of the study was to identify the role of adenosine deaminase (ADA) activity in patients with visceral leishmaniasis (VL) for management. There was a significant increase in mean ADA activity in sera of 49 patients with VL (323.71+/-184.51 IU/L) compared with 50 samples of control groups (47.11+/-24.94 IU/L) from the same endemic area (P < 0.001). ADA activities were found to be significantly decreased (50.35+/-41.35 IU/L) in follow-up cases (n = 19) after 30 days with sodium stibogluconate treatment at a dose of 20 mg/kg/day intramuscularly. The fall in the level of ADAF (after treatment) in follow-up cases correlated with the cure of disease, as evident from improvement of vital signs and symptoms and the absence of Leishmania donavani bodies in the sera. The study therefore suggests the possibility of using human serum ADA as a convenient marker to evaluate the diagnosis of VL to support the clinical findings, especially in those settings where there is a lack of highly qualified personnel and diagnostic facilities.