Background: Hematological abnormalities are common manifestations of advanced HIV-1 infection that could affect the outcomes of highly-active antiretroviral therapy (HAART). Although most HIV-1-infected individuals live in resource-constrained countries, there is little information about the frequency of hematological abnormalities such as anemia, neutropenia, and thrombocytopenia among individuals with advanced HIV-1 disease. Methods: This study compared the prevalence of pre-antiretroviral therapy hematological abnormalities among 1571 participants in a randomized trial of antiretroviral efficacy in Africa, Asia, South America, the Caribbean, and the USA. Potential covariates for anemia, neutropenia, and thrombocytopenia were identified in univariate analyses and evaluated in separate multivariable models for each hematological condition. Results: The frequencies of neutropenia (absolute neutrophil count ≤1.3×109/l), anemia (hemoglobin ≤10g/dl), and thrombocytopenia (platelets ≤125×109/l) at initiation of antiretroviral therapy were 14%, 12%, and 7%, respectively, and varied by country (p<0.0001 for each). In multivariable models, anemia was associated with gender, platelet count, and country; neutropenia was associated with CD4+ lymphocyte and platelet counts; and thrombocytopenia was associated with country, gender, and chronic hepatitis B infection. Conclusions: Differences in the frequency of pretreatment hematological abnormalities could have important implications for the choice of antiretroviral regimen in resource-constrained settings. © 2010 International Society for Infectious Diseases.