While the introduction of biomedicine within the Sri Lankan healthcare system has resulted in reduced reliance on traditional, complementary and alternative medicine (TCAM) over the last century in Sri Lanka, treatment modalities such as Ayurveda, Sinhala and traditional religious practices still receive considerable public support. Cancer is an increasing burden in Sri Lanka, and whilst Sri Lankans are known to use TCAM for everyday ailments, there has been no research examining the role of TCAM in the context of cancer. The aim of this study was to evaluate the prevalence and patterns of TCAM use by cancer patients. The study design was a cross-sectional survey. Data were obtained from two Sri Lankan hospitals using a consecutive convenience sample of 500 cancer patients currently receiving biomedical treatment, over a 10-week period in 2008. Analyses showed that 67.4% of those surveyed used one or more TCAM in conjunction with biomedicine for cancer treatment. The most common were Sinhala, Ayurveda and traditional religious practices. Of those patients who used TCAM, 95.0% gave the main reason for usage as ‘they thought it would cure their cancer’. The strongest reason for not using TCAM was on advice from their doctor (80.5%). The high use of TCAM amongst cancer patients in Sri Lanka raises numerous important issues, including those related to patient safety, potential benefit, interactions with biomedical cancer treatments, and delays in seeking biomedical cancer care. Further research is needed to explore the decision-making processes of cancer patients, including the perceived benefits/limitations of key processes in biomedical and TCAM care.