To help determine whether there is a genetic basis to the substantial variability observed in nail-patella syndrome (NPS), we devised a scoring system that quantifies the severity of the orthopaedic characteristics in NPS. Use of this system to score affected members in three generations of a single kindred revealed wide variability of severity of orthopaedic findings both within and between generations. Genetic testing in this family supported, but did not prove, a previously reported theory that the severity of the NPS in the offspring is modulated by the allele contributed by the unaffected parent. Evaluation of nonorthopaedic characteristics revealed the presence of glaucoma and the absence of kidney disease in this family. It is important that patients with NPS be evaluated for renal disease and glaucoma.