Hermansky-Pudlak syndrome (HPS) is an autosomal recessive disorder causing oculocutaneous albinism and a platelet storage pool deficiency, reflecting defective biosynthesis and/or processing of melanosomes and platelet dense bodies. Four human genes (HPS1, ADTB3A, HPS3, HPS4) are associated with four subtypes of HPS. The most common is HPS-1. A 16-bp duplication in exon 15 of the HPS1 gene causes HPS-1 in 450 northwest Puerto Rican patients; 13 other HPS1 mutations have been reported in non-Puerto Rican patients. We screened 26 HPS patients, who lacked a molecular diagnosis, for HPS1 defects and identified six patients with six different HPS1 mutations. Four novel mutations were discovered, including the first HPS1 missense mutation, 922T>C, in exon 8. This mutation, along with 624delG in exon 6, preserve RNA transcription, while 561delC in exon 5 and [1581delA;1594C>A] in exon 14 produce no RNA on northern blot. One of six adult patients developed pulmonary fibrosis, and two patients ages 16 and 17 have granulomatous colitis. These complications are common among Puerto Rican HPS-1 patients but have not appeared in HPS-2 or HPS-3 patients. The diagnosis of HPS-1, available only on molecular grounds, has important prognostic and treatment implications.