Identifying sources of variability in the response to cancer chemotherapy requires knowledge of all variables, including concomitant medications, that can alter the metabolism and pharmacokinetics of chemotherapy. This study investigated the accuracy of the lists of concomitant medications in the charts of cancer patients. Information collated from a questionnaire, patient interview, and the patient's medical chart was used to obtain validated medication lists. Patients took an average of 4.8 prescription drugs, 1.6 nonprescription drugs, and 1.6 other remedies within the 3 days prior to chemotherapy. Of the concomitant drugs actually taken, the medical records did not report 24% of prescription drugs, 84% of nonprescription drugs, and 83% of other remedies. Electronic medical records (EMRs) were more complete than paper charts, but even these omitted >75% of nonprescription drugs and other remedies. Potential drug interactions were noted in this study. This study documents the extent and complexity of the concomitant drugs taken by patients undergoing chemotherapy and the deficiencies in recording this information in medical charts.