To determine the frequency of opportunistic parasites, especially causative agents of microsporidiosis and cryptosporidiosis in HIV-infected patients in the Czech Republic, more than 75% of all Czech HIV-infected patients were examined during this study. Target staining techniques were used for parasitological examination of stool, sputum and urine of HIV-infected patients. In addition, their sera were examined by indirect ELISA technique with specific antigens from E. cuniculi, E. hellem and C. parvum. Specific antibodies to T. gondii were detected by CF test. In 2.1% of HIV-infected patients microsporidia E. bieneusi and E. intestinalis were found by parasitological examination, 5.3% of HIV-infected patients were seropositive to E. cuniculi and 1.3% to E. intestinalis were found by parasitological examination, 5.3% of HIV-infected patients were seropositive to E. cuniculi and 1.3% to E. hellem antigens in the high titre of 600. All blood donors (control group) were seronegative in these titres. C. parvum oocysts were found in one AIDS patient, 10.5% patients were seropositive to C. parvum in higher titres (600, 1800). Occurrence of T. gondii antibodies was not significantly greater in HIV-seropositive patients (27.5%) than in the control group (21.4%). P. carinii was detected in 13.6% of the patients. This study shows the quite low occurrence of opportunistic parasitoses among Czech HIV-infected patients. Although the high prevalence of specific antibodies against causative agents of microsporidiosis and cryptosporidiosis was recorded, the number of clinical cases was low.