The cause of hyperinsulinaemia in polycystic ovary syndrome (PCOS) is unknown, but two recent reports have implicated mutations of the tyrosine kinase domain of the insulin receptor gene in two patients. We have undertaken amplification of the insulin receptor gene using the polymerase chain reaction and single-stranded conformational polymorphism analysis in 22 hyperinsulinaemic patients with PCOS. Of these patients, 50% were polymorphic in exon 17 of the insulin receptor gene, but none of the alterations in sequence has been associated with insulin resistance. The genomic sequences in exons 18-21 were normal in all patients. We conclude that mutations involving the tyrosine kinase domain of the insulin receptor gene are a rare cause of insulin resistance in the PCOS.