Sera from 164 patients with parasitologically confirmed kala-azar and 100 patients with non-kala-azar Delhite in 2 Delhi hospitals were tested for anti-human immunodeficiency (anti-HIV) and anti-hepatitis C virus (anti-HCV) antibodies and hepatitis B surface antigens to determine which group is more likely to contract these infections. The mean age of the patients was 32.5 y (+/-6.5 y), (120 M, 44 F). Two patients were from Nepal and the others from the kala-azar endemic state of Bihar, India. As geographical controls, 50 serum samples from sex- and age-matched healthy Bihar residents were also tested for the blood-borne viral infections. All patients had been treated with injectable medicines by 1 or more local physicians before they were referred to the Delhi hospitals. The prevalence of hepatitis B virus (HBV) and HCV infection was significantly different between the 2 patient groups. While 2 kala-azar patients (1.21%) were found to be HIV-1 positive, 54 (32.9%) patients had anti-HCV antibodies detected by ELISA and 51 (31.1%) by RIBA test. The seroprevalence of HCV was only 2% in hospitalized non-kala-azar cases and 4% in the geographical controls (p < 0.001). The seroprevalence of HBV was 13.2% in hospitalized kala-azar cases, but only 1.75% in disease control cases and 1.6% in geographical control cases. The difference in infection rates between cases and controls was significant (p < 0.001). The results indicate that kala-azar patients treated locally in Bihar have a greater chance of contracting blood-borne infections. Interestingly, we found that HCV was more prevalent than HBV. These infections were most likely acquired through the re-use of needles by local medical and paramedical practitioners for administering anti-leishmanial drugs. This trend, if not checked immediately, may have drastic consequences in the horizontal transmission of HIV in Bihar.