Although anemia is widely considered an early sign of malignant disease, little is known about the pattern of hemoglobin decline before diagnosis. As an approach to understanding the duration of the preclinical phase of different types of malignant diseases, we investigated prediagnostic hemoglobin concentration changes in a large cohort of blood donors. Using a nested case-control design, we analyzed a population-based cohort comprising 1.1 million Scandinavian blood donors with complete follow-up through record linkage to population and cancer registers. A total of 16,375 cancer cases were identified, for whom we selected 161,995 controls. We used conditional logistic regression to estimate the risk of cancer in relation to hemoglobin concentration during the 5 years preceding the cancer diagnosis. Hemoglobin concentration decline began already 3 years before diagnosis of stomach cancer, multiple myeloma, and lymphatic leukemia; 2 years before diagnosis of small intestinal and colon cancer as well as of Hodgkin lymphoma. A decline was evident during the last year for non-Hodgkin lymphoma and myeloid/monocytic leukemia, whereas no change was found for cancer of the esophagus, breast or prostate. In conclusion, in this study, we have demonstrated that the pattern of declining hemoglobin concentration before cancer diagnosis varies considerably between malignancies without being a suitable screening tool for any of them. For some malignancies, however, the long duration of hemoglobin decline before clinical diagnosis suggests a substantial lead-time with systemic effects, during which earlier diagnosis should be achievable by emerging diagnostic tools.