Homocarnosinosis is a metabolic defect characterised by progressive spastic diplegia, intellectual deficit and retinitis pigmentosa. This extremely rare disorder has been reported in only one family, namely a woman and three of her children. The latter showed progressive spastic diplegia, mental retardation and retinitis pigmentosa but their mother was symptom free. It is therefore uncertain whether there is a relationship between the biochemical defect and the clinical symptoms. Inheritance in the reported family seems to be autosomal dominant.