We investigated serum levels of adenosine deaminase 2 (ADA2) and neopterin (NP) in hemophiliacs with or without infection with human immunodeficiency virus type 1 (HIV-1). The mean (+/- SD) serum ADA2 level in hemophiliacs positive for HIV-1 (45.2 +/- 17.6 U/L) and negative for HIV-1 (34.9 +/- 15.8 U/L) was significantly higher than that in healthy controls (12.0 +/- 7.0 U/L) (P < .01). The mean serum NP level was also higher in HIV-1-positive hemophiliacs (10.2 +/- 6.1 nmol/L) and HIV-1-negative hemophiliacs (7.0 +/- 2.9 nmol/L) than in the healthy controls (4.3 +/- 1.3 nmol/L). Although the HIV-1-positive hemophiliacs had higher mean ADA2 and NP levels than did hemophiliacs in the HIV-1-negative group (P < .01), the levels of most of the patients in both groups were similar. ADA2 and NP levels in serial samples from asymptomatic carriers and patients with stable AIDS showed no marked changes over a period of up to 6 years. These findings indicate that ADA2 and NP are not specific markers of HIV-1 infection in hemophiliacs. Nonspecific immunologic activation due to the repeated infusion of antihemophilic factor concentrate could be one cause for the increased serum levels of ADA2 and NP in hemophiliacs.