Sera from 38 HIV-infected individuals were examined longitudinally for antibodies to viruses that may increase morbidity in HIV infection, as well as commensal viruses and Toxoplasma gondii. HTLV infection was seen in Norway for the first time as four patients had antibodies to HTLV-II and one had antibodies to HTLV-I. Antibodies to hepatitis B virus (HBV) were found in 47.2%, while 21.6% of the patients had antibodies to hepatitis C virus (HCV). There was no evidence of acquisition of HBV or HVC during the mean observation period of 2 years. A titre increase in CMV antibody with time was observed for 7 out of 21 patients and a decrease for 2 patients. For Epstein-Barr virus, herpes simplex, varicella-zoster, rubella and measles viruses, human polyomavirus BK as well as for Toxoplasma gondii, antibody prevalences and titres were within the range seen in normal populations. Also, no longitudinal changes were observed in titres of these antibodies, indicating that humoral immunity remained intact during the study period. The high prevalences of HTLV-I/II, HBV and HCV antibodies in HIV-infected patients reflect common modes of virus transmission, and the fluctuations in CMV antibody titre are indicative of reactivations. Such coinfections may influence disease progression.