To assess nationally the use of questionable cancer therapies. Survey of cancer patients or their families regarding use of questionable methods of cancer treatment; sample survey of physicians' perception of use. 36,000 households; 5,047 individuals; 91 physicians. The prevalence of use of questionable cancer methods was nine percent overall. An increase in use was directly proportional to increased income and education. Prolonged illness and certain types of cancer were more commonly associated with use. Harmful side effects of questionable cancer treatments were regarded as modest (six percent). There was a wide range in cost. Third-party reimbursement was reported by 25 percent of patients. Important discrepancies were found between patients' and physicians' perceptions of questionable therapies. While some questionable therapies are harmless or inexpensive, others have toxic effects and may be costly, and none have scientifically proven efficacy. Although the percentage of usage reported is relatively low, overall large numbers of patients are involved, especially in certain groups. The physician plays a key role in encouraging or preventing the use of questionable methods, and substantial improvements in public and professional education are needed.