To determine the frequencies of beta-thalassemia mutations in different states of India and to compare this with the available data in Asian Indians for a comprehensive catalogue of molecular defects in the Indian population. beta-thalassemia mutations were characterized in 2456 heterozygotes using reverse dot blot hybridization, ARMS and DNA sequencing. 36 beta-thalassemia mutations were characterized from 18 different states in India. Seven mutations were common, accounting for 95.8% of mutated alleles. Marked regional diversity was seen in different parts of the country. Among the tribal populations, only 2 mutations (IVS I-5 (G-->C) and CD15 (G-->A) accounted for over 90% of mutant alleles. A compilation of all the studies in Asian Indians reported so far showed the presence of 63 mutations in the Indian population. This large study adds to the existing data to give a detailed account of the molecular basis of beta-thalassemia in India. This information is important for establishing prenatal diagnosis programmes in different states in India as well as other countries in which there is a major influx of Indian immigrants.