Eosinophilia and anemia are very common hematological alterations in the tropics but population-based studies scrutinizing their value for diagnosing parasitic infections are rare. Methodology: A cross-sectional study was conducted in a rural district in northeast Brazil where parasitic infections are common. Stool and blood samples were collected and individuals were clinically examined for the presence of ectoparasites. Results: In total, 874 individuals were examined. Infection with

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... l helminths occurred in 70% (95% CI 67 -75), infestation with ectoparasites in 45% (95% CI 42 -49) and co-infection with both helminths and ectoparasites was found in 33% (95% CI 29% -36%) of all inhabitants. Eosinophil counts ranged from 40/µl to 13.800/µl (median: 900/µl). Haemoglobin levels ranged from 4.8 g/dl to 16.8 g/dl (median: 12.5 g/dl), and anemia was present in 24% of the participants. Leukocytosis was found in 13%, eosinophilia in 74%, and hypereosinophilia in 44% of the participants. Eosinophilia was more pronounced in individuals co-infected with intestinal helminths and ectoparasites (p < 0.001) and correctly predicted parasitic infection in 87% (95% CI 84%-90.7%) of all cases. Conclusions: Eosinophilia is strongly associated with the presence of intestinal helminthiases and accentuated by co-infestation with ectoparasites. Our study confirms in a population with high prevalence of intestinal helminthiases and ectoparasites that eosinophilia can be used to accurately diagnose current parasitic infection and initiate treatment.