Oncologists are aware that their patients use complementary/alternative medicine (CAM). As cancer incidence rates and survival time increase, use of CAM will likely increase. This study assessed the prevalence and predictors of CAM use in a comprehensive cancer center. Subjects were English-speaking cancer patients at least 18 years of age, attending one of eight outpatient clinics at The University of Texas M.D. Anderson Cancer Center, Houston, TX, between December 1997 and June 1998. After giving written informed consent, participants completed a self-administered questionnaire. Differences between CAM users and nonusers were assessed by chi(2) and univariate logistic regression analysis. A multivariate logistic regression model identified the simultaneous impact of demographic, clinical, and treatment variables on CAM use; P values were two-sided. Of the 453 participants (response rate, 51.4%), 99.3% had heard of CAM. Of those, 83.3% had used at least one CAM approach. Use was greatest for spiritual practices (80.5%), vitamins and herbs (62.6%), and movement and physical therapies (59.2%) and predicted (P <.001) by sex (female), younger age, indigent pay status, and surgery. After excluding spiritual practices and psychotherapy, 95.8% of participants were aware of CAM and 68.7% of those had used CAM. Use was predicted (P <.0001) by sex (female), education, and chemotherapy. In most categories, CAM use was common among outpatients. Given the number of patients combining vitamins and herbs with conventional treatments, the oncology community must improve patient-provider communication, offer reliable information to patients, and initiate research to determine possible drug-herb-vitamin interactions.