The objective of our study was to determine the production of nitric oxide (NO) in patients infected with human immunodeficiency virus (HIV) and its relation to cellular immunity, activation of mononuclear phagocytes and the amount of circulating virus. Therefore, serum nitrate, the stable metabolite of NO, the number of peripheral CD4+ T-lmphocytes, serum neopterin, plasma HIV-RNA and HIV-DNA in peripheral blood mononuclear cells of afebrile HIV-infected patients were determined. Serum nitrate levels were significantly (p = 0.002) increased in HIV-infected patients (median 37 microM, range 13-137 microM, n = 77) in comparison to healthy subjects (median 28 microM, range 21-40 microM, n = 17). Serum nitrate levels did not correlate with the number of CD4+ T-lymphocytes (r = 0.05, p > 0.05). Serum nitrate levels were positively correlated with neopterin (r = 0.36, p = 0.05, n = 30), the amount of HIV-DNA in peripheral blood mononuclear cells (r = 0.63, p < 0.001, n =27) and plasma HIV-RNA levels (r = 0.35, p = 0.08, n = 27). A possible explanation of our findings is that HIV induces the production of NO by means of activated mononuclear phagocytes.