Little is known regarding the prevalence of outpatient cancer treatment in the U.S. We analyzed nationally-representative data from the 2000-2004 Medical Expenditure Panel Survey to estimate the number of U.S. cancer patients receiving outpatient chemotherapy and/or radiation therapy annually. Each year, over 1.1 million individuals are estimated to receive chemotherapy or radiation therapy for cancer. Cancer patients younger than 65 receiving treatment who were uninsured were less likely to receive chemotherapy or combined chemotherapy/radiation therapy than were those with public or private insurance. These estimates may be useful for understanding the burden of cancer care and development of programs for cancer survivors.