A cross-sectional study was undertaken to determine the prevalence of isosporiasis and its clinical and laboratory pattern in Venezuelan patients infected with human immunodeficiency virus (HIV) (n = 397). At enrollment, they underwent a thorough clinical history and physical examination, and provided stool specimens for the identification of Isospora belli and other parasites. Isospora belli was identified in 56 subjects (14%) and diarrhea, either acute or chronic, was present in 98% of these cases (P < 0.001). Eosinophilia was strongly associated with isosporiasis (P = 0.01). It was also found that the presence of eosinophilia was more common in I. belli-infected patients without weight loss (P < 0.001). Twenty-six (81.25%) subjects with I. belli infection had CD4+ cell counts < 200 cells/mm3 (P = 0.03). In addition, the data and its description shows the association to be < 100 cells/mm3. This infection seems to be seasonal because the recovery of oocysts occurred mainly in months with significant rainfall. In fact, isosporiasis should be suspected in HIV-infected patients from tropical countries with diarrhea, weight loss, eosinophilia, and low CD4+ cell counts.