OBJECTIVE: To examine the association between hemoglobin levels and anemia diagnosis and to determine the hemoglobin level that is most likely to trigger diagnosis in oncology patients. METHODS: Members of a large US health plan with an oncology diagnosis and chemotherapy claims between 1/1/2000 and 2/28/2002 were included in this retrospective claims database analysis. Medical and laboratory claims were examined to identify chemotherapeutic episodes, International Classification of Diseases 9th Modification (ICD-9) codes for anemia, and hemoglobin values within each episode and immediately preceding new anemia diagnoses. Descriptive statistics and multivariate regression were used to examine the relationship between anemia diagnosis and hemoglobin values. RESULTS: A total of 3180 chemotherapeutic episodes corresponding to 2717 oncology patients were identified. In episodes in which the hemoglobin dropped below 12 g/dL (1,689 episodes; 53%), an anemia diagnosis occurred in only 733 episodes (45%). Additionally, an anemia diagnosis was found in only 66% of the episodes where hemoglobin fell below 10.0 g/dL. Being over 50 years old, having Non-Hodgkins Lymphoma, and having fatigue or renal disease were observed to increase the odds of diagnosis controlling for hemoglobin nadir values and chemotherapeutic agent. Adjusting for age above 50 years old, gender and breast cancer, the mean hemoglobin value prior to anemia diagnosis was 11.2 g/dL (95% CI; 11.0, 11.3). CONCLUSION: These data suggest that many patients who become anemic during chemotherapy (> Grade 1 anemia) do not receive an anemia diagnosis. At hemoglobin values below 10 g/dL, the level at which the National Comprehensive Cancer Network (NCCN) most strongly recommends consideration of aggressive anemia treatment, only two-thirds of patients were identified as anemic by an ICD-9 code. Our findings provide evidence that identifying patients with ICD-9 codes for anemia from claims data may be misleading and can significantly underestimate the true number of patients with anemia.