This study aimed to assess the prevalence of complementary and alternative medicine (CAM) use in a representative cancer population prior to and within 6 months of diagnosis. A total of 304 newly diagnosed cancer patients from two UK cancer centres completed a postal survey. Of them, 100 patients (32.9%) used CAM before their cancer diagnosis, 59 of these CAM users continued post diagnosis. Twenty-nine individuals who had not used CAM before began to use it after their cancer diagnosis, creating a total of 88 (28.9%) CAM users in this sample. Reasons for not using CAM included lack of interest, lack of information or endorsement from professionals and satisfaction with conventional care. For those using CAM before diagnosis but not afterwards, the most common reason was a lack of expert guidance on what was safe to use. The use of CAM medicines bought from health food and other retail outlets was high. Complementary and alternative medicine use in cancer patients is common and demonstrates a complex pattern, but CAM use is not significantly greater than in the general population. Some patients purchase CAM medicines without seeking medical advice, thus risking drug interactions. Research to generate information on safety and efficacy of CAM is required.