This paper is a philological study of twelve Sri Lankan medical manuscripts preserved in the Bodleian Library, Oxford, and the Cambridge University Library. It shows the impact of Buddhism on the development of traditional medicine in South Asia and the important role played by South Indian Vaidyas in the propagation of medical knowledge in Sri Lanka. Those Vaidyas appear to have been well versed in Siddha medicine and proficient in both Sinhala and Tamil languages. Only the historical and cultural aspects revealed in these documents are dealt with here. The therapeutic aspect is no less important in view of the large number of medicinal prescriptions, some of which are said to be of proven efficacy. The material presented may be of interest to students of the history of medicine and medical anthropology.