Unilateral renal agenesis is an uncommon association with Kallmann's syndrome (KS) (hypogonadotrophic hypogonadism and olfactory defect). We have investigated affected individuals from six pedigrees: five with X-linked KS, and one with X-linked KS and X-linked ichthyosis (XLI). Seventeen affected individuals have had renal imaging performed, and six scans demonstrated only one kidney. In addition, two pedigrees had males who died in the neonatal period with bilateral renal agenesis. Only two of the four affected individuals in the family with X-linked KS and X-linked ichthyosis (Pedigree 6) showed unilateral renal agenesis, despite all four patients demonstrating an interstitial deletion within the short arm of the X-chromosome. These data indicate that unilateral renal agenesis is much commoner than previously suspected in patients with X-linked Kallmann's syndrome, but that it may have incomplete penetrance within a family.