We have previously demonstrated allele loss in hamartomas from patients with tuberous sclerosis for markers spanning the tuberous sclerosis gene on chromosome 16q13.3 (TSC2). Germline deletions in the TSC2 gene have been shown in 5% of patients with tuberous sclerosis (TSC). These data support our hypothesis that the TSC2 gene acts as a growth suppressor gene, analogous to the traditional tumour suppressor gene. We now report a TSC hamartoma showing allele loss for markers on chromosome 9q34 in the region of the TSC1 gene. We studied six hamartomas from four sporadic and two familial cases of TSC, none of which showed allele loss for markers on chromosome 16p13.3. The hamartomas were paraffin embedded sections of three renal angiomyolipomas, two giant cell astrocytomas, and a cardiac rhabdomyoma. Eight markers were analysed, comprising from centromeric to telomeric ASS-D9S64-D9S149-ABO-D9S150-DBH-D9S66-D9S67++ +. One angiomyolipoma showed allele loss for the markers ABO, DBH and D9S66, but not for D9S149 or D9S67. The patient was not informative for D9S150. The family structure did not permit the phase of the disease and marker alleles to be determined. These finding support the hypothesis that the TSC1 gene on 9q34, like the TSC2 gene, acts as a growth suppressor. The data would place the TSC1 gene between D9S149 and D9S67. Mapping of allele loss in hamartomas may help in the refinement of the location of the TSC1 locus.