Microsporidia are recognized as a cause of morbidity among patients infected with the human immunodeficiency virus (HIV). Infection rates for intestinal microsporidiosis in HIV-infected patients from Venezuela are unknown. To determine the prevalence and pathogenic role of microsporidia in these patients in northwestern Venezuela, a case control study was conducted in 103 outpatients (mean +/- SD age = 37.3 +/- 5.6 years). Microsporidia were detected using unconcentrated formalin-fixed stools examined by Weber's chromotrope-based staining method. For identification of coccidia, modified Ziehl-Neelsen carbolfuchsin staining of formalin-ether concentrates were used, and for other pathogenic parasites, iron hematoxylin-stained smears and formalin-ether concentrates were examined. Microsporidial infections were detected in 14 (13.6%) of 103 patients and 39 (37.9%) had other parasitic pathogens. No significant difference was noted in the occurrence of the infection in patients with diarrhea (13 of 74, 17.6%) and controls (1 of 29, 3.4%) (P = 0.118). Nevertheless, this result may be due to the small sample size (n = 14) of infected individuals. The proportions of other pathogens in patients with or without diarrhea were not significantly different (P = 0.828). Microsporidiosis is common among the HIV-infected population in northwestern Venezuela. However, its pathogenic role in these patients is uncertain and warrants further investigation.