Controversy exists regarding the ability of cancer chemotherapy to prevent the development of an adequate immune response to influenza virus vaccine. Of 12 studies addressing this issue, eight demonstrated a significant lessening of the immune response among patients receiving cancer chemotherapy. The other four studies failed to find a significant difference between the immune responses of patients receiving cancer chemotherapy and persons not receiving chemotherapy; for these studies the Type 2 error rate (i.e., the probability of wrongly concluding that no difference exists) was calculated. Since the response rates in the four inconclusive studies were consistent with those in the other eight studies but the sample sizes were much smaller, the failure of the former studies to find significant differences in immune responses probably was due to insensitivity rather than to the absence of such differences. The preponderance of evidence suggests that the serum antibody response to influenza virus vaccine is significantly weaker in patients receiving cancer chemotherapy than in persons not receiving chemotherapy.