Background. Eosinophilia may represent an early paraclinical sign of malignant disease and a host anti-tumor effect. The association between eosinophilia and the development of solid tumors has never before been examined in an epidemiological setting. The aim of the present study was to investigate eosinophilia in routine blood samples as a potential biomarker of solid tumor development in a prospective design. Material and methods. From the Copenhagen Primary Care Differential Count (CopDiff) Database, we identified 356 196 individuals with at least one differential cell count (DIFF) encompassing the eosinophil count during 2000-2007. From these, one DIFF was randomly chosen and categorized according to no (< 0.5 × 10(9)/l), mild (≥ 0.5-1.0 × 10(9)/l) or severe (≥ 1.0 × 10(9)/l) eosinophilia. From the Danish Cancer Registry we ascertained solid tumors within the first three years following the DIFF. Using multivariable logistic regression, odds ratios (OR) were calculated and adjusted for previous eosinophilia, sex, age, year, month, C-reactive protein, previous cancer and Charlson's comorbidity index. Results. The risk for breast cancer was significantly lower in individuals exhibiting mild eosinophilia than in individuals with normal eosinophil counts [OR (95% confidence intervals) = 0.51 (0.35-0.76), p = 0.0005]. The risk of bladder cancer, however, increased with severity of eosinophilia [2.27 (1.53-3.39), p < 0.0001 and 2.62 (0.96-7.14), p = 0.0592 for mild and severe eosinophilia, respectively]. No associations with eosinophilia were observed for remaining solid cancers. Conclusion. We demonstrate that eosinophilia in routine blood samples associate with a decreased risk of breast cancer and an increased risk of bladder cancer. Our data emphasize that additional preclinical studies are needed in order to shed further light on the role of eosinophils in carcinogenesis, where it is still unknown whether the cells contribute to tumor immune surveillance or neoplastic evolution.