The analysis of a sample of skeletons from the 4,000-year-old site of Khok Phanom Di on the coast of central Thailand has identified a number of individuals with skeletal evidence suggestive of severe anemia. The differential diagnosis of the lesions is discussed and the presence of one of the thalassemia syndromes is proposed. The implications of this for southeast Asian prehistory are discussed. The presence of these conditions has been suggested in previous analyses of prehistoric southeast Asian populations, but this is the first population in which the evidence, including postcranial responses, is presented in detail.