Tuberous sclerosis complex (TSC) is an autosomal dominant disorder caused by mutations in either the TSC1 or the TSC2 genes and characterized by the development of benign hamartomatous growths in multiple organ systems. We have inactivated Tsc1 in the mouse germ line by gene targeting in ES cells and confirmed that the mutant allele (Tsc1-) has a recessive embryonic lethal phenotype. We found that a significant number (approximately 27%) of heterozygous (Tsc1+/-) mice on the C57BL/6 background died before weaning (P = 0.014) and show that these mice die in the post-natal period (P = 0.033), normally at 1-2 days, from unknown causes. Forty-four percent (7/16) of Tsc1+/- mice on a C3H background developed macroscopically visible renal lesions as early as 3-6 months, increasing to 95% (37/39) by 15-18 months. Renal lesions progressed from cysts through cystadenomas to solid carcinomas. Eighty percent (16/20) of Tsc1+/- mice on a Balb/c background exhibited solid renal cell carcinomas (RCC) by 15-18 months and in 41%, RCCs were > or = 5 mm, resulting in grossly deformed kidneys. Some RCCs had a sarcomatoid morphology of spindle cells in whorled patterns and metastasized to the lungs. We detected loss of the wild-type Tsc1 allele and elevated levels of p-mTOR and p-S6 in lesions from Tsc1+/- mice. This new murine model of hamartin deficiency exhibits a more severe phenotype than existing models.