Previous linkage studies in tuberous sclerosis have implicated three disease determining loci at 9q, 11q, and 12q. We have collated phenotypic and genotypic data on 1622 members of 128 families with tuberous sclerosis in order to evaluate simultaneously the evidence for these putative loci. Affection status in the family members has been reassessed using uniform diagnostic criteria and genotypic data extensively checked before analysis under alternative models of locus heterogeneity. One tuberous sclerosis determining locus, accounting for approximately 50% of the families studied, has been found to map in the region of D9S10 on 9q34 but no evidence has been found to support the existence of major loci on 11q or 12q. A locus, or loci, elsewhere in the genome is likely to account for tuberous sclerosis in most non-chromosome 9 linked families.