Predictors of physician nonadherence to chemotherapy regimens. Physician nonadherence to cancer treatment regimens may diminish treatment efficacy and compromise clinical research. The influence of clinical, demographic, and psychosocial patient characteristics on physician adherence to breast cancer chemotherapy was investigated, as was the role of the clinician's attitudes concerning the chemotherapy. One hundred seven women recently diagnosed with breast cancer were followed for 26 weeks of treatment. Fifty-six (52%) of the patients experienced unjustified modification for at least one chemotherapeutic agent. Stepwise multiple regression revealed independent contributions of increased patient age, treatment setting (clinic versus academic or community private practice), and stage of disease to physician nonadherence. Regimen complexity, delay in seeking treatment, and presence of psychiatric disorder did not contribute, in general, to unjustified regimen modifications. Patient psychological and psychiatric factors, however, did influence prescribing behavior for vincristine. Physician awareness of factors contributing to unnecessary treatment modifications may reduce the frequency of such behaviors. 