Hereditary osteo-onychodysplasia (HOOD, nail-patella syndrome) is an autosomal dominant condition characterised by nail dysplasia, patellar hypoplasia or aplasia, and nephropathy. The risk for HOOD patients to have a child with HOOD who will develop renal failure cannot easily be deduced from published pedigrees. We have studied a large family with 30 patients with HOOD and have analysed 34 kindreds with HOOD nephropathy from published reports, comprising 213 patients. For a patient with HOOD from a family in which HOOD nephropathy occurs, the risk of having a child with HOOD nephropathy is about 1:4; the risk of having a child in whom renal failure will develop is about 1:10.