Tuberous sclerosis (TSC) is an autosomal dominantly inherited multisystemic disease characterized by the development of hamartomas predominantly in brain and kidneys. The TSC2 gene for tuberous sclerosis is localized on chromosome 16p13.3 immediately adjacent to PKD1, the gene for autosomal dominant polycystic kidney disease (ADPKD). A TSC2-PKD1 contiguous gene syndrome caused by chromosomal microdeletions disrupting both the TSC2 and PKD1 genes has been identified in patients with TSC and early-onset severe ADPKD. We report a 3-month-old Caucasian girl of non-consanguineous parents with TSC and early manifestation of ADPKD. She presented with right-sided focal seizures, two small hypopigmented areas on the left flank, and elevated blood pressure requiring antihypertensive treatment. Brain magnetic resonance imaging revealed typical signs of tuberous sclerosis and abdominal ultrasonography showed bilaterally enlarged kidneys with multiple cysts resembling those seen in ADPKD. There was no family history of renal disease or of tuberous sclerosis. Findings were highly suspicious of TSC2-PKD1 contiguous gene syndrome. Using fluorescence in situ hybridization and plasmid probe CW23, which spans the adjacent 3' regions of TSC2 and PKD1 genes, we identified a submicroscopic deletion on only one of the chromosomes 16p13.3, thus permitting the diagnosis of the TSC2-PKD1 contiguous gene syndrome.