Eosinophilia may represent an early paraclinical sign of hematological malignant disease, but no reports exist on its predictive value for hematological malignancies. 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 x 10(9) /l), "mild" (≥0.5-1.0 x 10(9) /l) or severe (≥1.0 x 10(9) /l) eosinophilia. From the Danish Cancer Registry and the Danish Civil Registration System we ascertained hematological malignancies and death within three years following the DIFF. Using multivariable logistic regression odds ratios (ORs) were calculated and adjusted for previous eosinophilia in a DIFF, sex, age, year, month, C-reactive protein, previous cancer and comorbidity. ORs for developing Hodgkin's lymphoma (HL) was significantly increased in individuals exhibiting severe eosinophilia, OR=9.09 (C.I. 2.77-29.84), P=0.0003. The association with classical myeloproliferative neoplasms (cMPNs) showed an increasing risk with OR=1.65 (1.04-2.61) P=0.0322 and OR=3.87 (1.67-8.96) P=0.0016) for mild and severe eosinophilia. Eosinophilia was in a similar fashion associated with chronic lymphatic leukemia (CLL), OR=2.57 (1.50-4.43), P=0.0006 and OR=5.00 (1.57-15.94), P=0.0065, and all-cause death, OR of 1.16 (1.09-1.24), P<0.0001 and 1.60 (1.35-1.91), P<0.0001. We confirm associations between eosinophilia and HL and cMPNs, and in addition for the first time demonstrate a dose-dependent association between eosinophilia and CLL as well as death. Unexplained eosinophilia should prompt clinicians to consider conditions where early diagnosis may improve prognosis.