Levels of anticardiolipin antibodies (ACA) were measured in 55 patients with haemophilia A in serum samples obtained in 1983 and in 1987. Twenty-one patients were negative for anti HIV-1 antibodies in 1983 and remained negative in 1987; 34 patients had anti HIV-1 antibodies in 1983; 17 of these latter patients remained asymptomatic, whereas 17 patients developed ARC or AIDS during the 4 years follow-up. Thirteen anti HIV-1 negative patients had elevated ACA levels in 1983; subsequently, a significant decrease was observed in all these subjects (p less than 0.001). All anti HIV-1 positive patients had elevated ACA levels in 1983; normal values were found in 9 patients in 1987. Yet, these changes were not significant (p greater than 0.05). ACA levels were significantly higher in HIV-1 infected patients than in those without anti HIV-1 antibodies (p less than 0.05). There was no difference of ACA levels between the two anti HIV-1 positive patient groups, be it in 1983 or be it in 1987 (p greater than 0.05). There was no correlation of ACA levels with serum IgG concentrations, CD4+ lymphocytes, or the consumption of factor VIII concentrates.