Retinopathy of prematurity (ROP) is a leading cause of blindness in premature children. It is a multifactorial disorder which causes fibrovascular tissue changes that affect the retina in low birth-weight and short gestational age infants. To determine the prevalence of Norrie disease (ND) gene mutations, clinical examination and molecular genetic analyses were performed in 100 pre-term babies of different ethnic backgrounds who developed advanced ROP. The leukocyte DNA was extracted, amplified by the polymerase chain reaction (PCR), and analyzed by single-strand conformation polymorphism (SSCP), G/T and C/A scanning, and by DNA sequencing. All three exons, including splice sites and the 3'-untranslated region, were screened. Of the 100 patients analyzed, 2 patients with advanced ROP showed a mobility shift in the DNA. In 1 patient, this mobility shift was caused by the insertion of an additional 12-bp CT repeat in exon 1, and in the second patient, there was a 14-bp deletion in the same exon of the ND gene, as evidenced by direct sequencing of the amplified products. Similar analyses of exons 2 and 3 and the 3'-untranslated region failed to detect additional mutations in the gene. None of the 130 normal, unrelated controls revealed similar changes. Taking into account the above results, as well as those of other studies, it appears that the ND gene mutations can account for 3% of cases of advanced ROP. Although the ND gene is not frequently involved in advanced ROP, the present large-scale study further supports the hypothesis that genetic influences may play an important role in the development of severe ROP in some premature infants.