During a 20-month period, more than 500 splenic aspirations were performed in 89 patients with suspected or proven visceral leishmaniasis. The two complications which occurred (intra-abdominal bleeding and penetration of the intestine in one patient each) both resolved with conservative management. Parasite density in splenic aspirate smears was graded on a logarithmic scale from 0 (no parasites in 1,000 microscopic fields) to 6+ (greater than 100 parasites per microscopic field). Among 46 newly diagnosed and 17 relapsed or drug-resistant patients with visceral leishmaniasis, the average initial parasite grade was 4.35 +/- 0.92 (mean +/- SD) and 4.15 +/- 1.37, respectively. The grading system was useful in measuring the speed of response to treatment, and in distinguishing slow responders from nonresponders. This was especially valuable for managing patients with drug-resistant visceral leishmaniasis. The system also provided a means of comparing the efficacy of different treatment regimens, and for calculating the optimum duration of treatment.