IgG and IgM anti-cardiolipin antibodies were measured, by an ELISA technique, in the sera of patients with B hepatitis (28), infectious mononucleosis (10), chicken pox (12), HIV infection (20), acquired toxoplasmosis (41) and other infectious diseases [HBsAg+ chronic hepatitis (5), brucellosis (6), herpes zoster (4), boutonneuse fever (3), viral pneumonitis (4), rheumatic fever (2)]. Increased levels of anti-cardiolipin antibodies (aCL), at least in one immunoglobulin class, were detected in 37 out of 135 patients [27.4%; range: 7.3% (in the patients with toxoplasmosis) -80% (in the patients with HIV infection)]. Low or medium titer aCL were present in 28 patients, high titer in 9 (6 with HIV infection, 2 with chicken pox and I with lymphoadenopathic toxoplasmosis). None of the manifestations associated with aCL was present in the aCL-positive patients. Finally, positivity for aCL didn't seem to modify the clinical picture and the prognosis of the infectious disease.