Molecular analysis of four unrelated patients with Lesch-Nyhan (L-N) syndrome was performed. All four cases had typical clinical features of L-N syndrome, and the activities of hypoxanthine-guanine phosphoribosyltransferase (HPRT) were absent. No structural gene abnormalities were found by Southern blot analysis in all cases, and a decrease of HPRT mRNA was not detected by dot blot analysis in two of the four cases. However, HPRT enzyme proteins were detected by Western blot analysis in all cases. Our results showed that the production of immunologically reactive but enzymatically inactive mutant HPRT protein was pathogenic for all four patients.