Eleven possible and five obligate carriers of Lowe's syndrome from the same pedigree were examined for lens opacities. All of the obligate carriers and 4 of the 11 at risk had lens abnormalities. The lenticular abnormalities consisted of cortical dots of various shapes that increased in number with the age of the carriers in older obligate carriers, subcapsular plaques were common. Because the syndrome is X linked, such lens changes are explainable by the Lyon's hypothesis. When the number of opacities seen in these subjects were compared to those seen in 100 normal control females 10 to 20 years of age, cataractagenic cases such as diabetes, Down's and fetal nuclear opacities excluded, four probable carriers were identified among the eleven possible carriers in the pedigree. These subjects had significantly greater numbers of opacities, similar to those seen in obligate carriers, compared to controls. We conclude that progressive lens changes are present in carriers of Lowe's syndrome and that young carrier females can be identified reliably when they are compared to age-matched controls by modifying the grading system of Brown and Gardner.