G6PD deficiency may render afflicted individuals more susceptible to certain degenerative diseases. To clarify the relationship between G6PD deficiency and cataract, blood G6PD activity was determined in patients with cataract in Taiwan. The cases and hospital-based controls were recruited from the medical outpatient department and from the physical checkup department at Chang Gung Memorial Hospital, respectively. A questionnaire survey was used to determine associations between cataracts and their risk factors. G6PD activity in fresh RBC was quantitatively measured and genomic DNA was extracted from lymphocyte nuclei. The mean blood G6PD activity among cataract patients (278.1 U/10[12] RBC) was similar to that of normals (288.0 U/10[12] RBC). No statistically significant difference in the distribution of G6PD activities as grouped by an increment of 100 U/[10, 12] RBC was observed between cataract patients and normal subjects. The predominant forms of G6PD gene mutation (cDNA 1376 G to T and 95 A to G) were both found in the patients with cataract. The adjusted odds ratio for cataract was 1.21 for every decrement of 100 U/10[12] RBC of G6PD activity in these subjects. These data indicate that G6PD activity is not a potential risk factor for senile cataract in Taiwan.