No association between breast cancer and treatment with neuroleptic agents agents was detected in a study of 5463 women who died or underwent breast biopsy at the Norristown State Hospital between 1940 and 1974. The method of proportionality was used to evaluate the frequency with which breast cancer occurred at the hospital; cases of breast cancer were related to deaths from other cancers and to deaths from all causes other than breast cancer. Deaths and cancers that occurred within 1 year of hospital admission were excluded from the analysis. To allow for a 10-year transition period and for latency in the development of breast cancer, comparison was made between the 15 years preceding the introduction of neuroleptic agents into medical practice (1940 to 1954) and the second decade after the introduction (1965 to 1974). Although neuroleptic agents were used extensively for treatment of patients at the hospital, there was no statistically or medically significant change in the indicated incidence of breast cancer at the hospital during the neuroleptic period. The study did not rule out the possibility that neuroleptic agents might have some effect on breast cancer after a very long latency period.