A study of albinism in Puerto Rico identified 693 persons with albinism. Among these, the type of albinism was determined in 595, Hermansky-Pudlak syndrome (HPS) was found in 495. Approximately five of every six Puerto Rican albinos had HPS. The highest prevalence of HPS yet reported was in the northwestern quarter of the island where at least 1 in 1,800 persons had HPS, and approximately 1 in 21 were carriers. The HPS albino pigment phenotype was variable, and HPS albinos phenotypically resembled other types of oculocutaneous and ocular albinos. Ceroid storage was also variable. The consistent finding in HPS was storage pool deficient platelets. HPS is best diagnosed by lack of platelet dense bodies seen by electron microscopy. Evidence from family studies indicates that HPS is a distinct disorder due to the pleiotropic effects of a single gene mutation or a small deletion.