The decrease in testosterone production associated with bilateral orchiectomy may result in normocytic anemia in men. We sought to determine the effect of orchiectomy on hemoglobin concentration. Patients were evaluated at the Mayo Clinic in 1993 and 1994 and underwent bilateral orchiectomy for prostate carcinoma. All patients were seen by one of the staff urologists. Patients were included if they had a normal preoperative complete blood cell count and serum levels of creatinine, if they remained without disease progression (suppressed prostate-specific antigen level and no evidence of clinical progression on review), and if they had normal serum levels of creatinine and mean corpuscular volume during the follow-up period. The patients could have no other cause of anemia. The complete blood cell count, prostate-specific antigen level, and serum level of creatinine were determined preoperatively and at least once (>90 days) after orchiectomy. Sixty-four patients were included in the analysis (median age, 68 years). The median decrease in hemoglobin concentration was 1.2 g/dL after orchiectomy. There was a statistically significant difference in the hemoglobin concentration before orchiectomy compared with postoperative values at all the intervals studied, both by the paired group t-test and the Kruskal-Wallis test. There is a clinically and statistically significant decrease in hemoglobin concentration after orchiectomy. Knowledge of this phenomenon may prevent unnecessary diagnostic work-up in men with normocytic anemia after bilateral orchiectomy. Am.