In a Hawaii Hereditary Anemia Screening Project, 4,984 participants were tested for glucose-6-phosphate dehydrogenase (G6PD) deficiency by a filter paper blood spot fluorescence test. Abnormal samples and suspected heterozygotes were checked by quantitative G6PD assay (normal 4.5 to 14 units/g Hb). G6PD was deficient (< 1.5 units/g Hb) in 188 of 2,155 males; 7 other males had low activity (1.5 to 2.8 units/g Hb). The gene frequency, estimated from males after excluding referred and related cases, was 0.037 for Chinese, 0.134 for Filipinos, and 0.203 for Laotians. Among 2,829 females tested, family data showed 111 females were obliged to be at least heterozygous, regardless of G6PD activity, and 43 others had low G6PD activity. Most heterozygotes probably remained undetected by G6PD screening. In 28 females, activity was under 10%; in another 9 females, activity was < 1.5 units/g Hb. Since only 25 homozygotes would be predicted, this apparent excess of females with deficient activity could be due to unequal X-inactivation in some heterozygotes. DNA analysis by polymerase chain reaction amplification and special analytic procedures revealed 10 different missense mutations in 75 males. The nucleotide 835 A-->T and 1360 C-->T transitions were first detected in this Hawaiian Project; we found that the nucleotide 1360 mutation was the most common cause of G6PD deficiency in Filipinos. This is the first report of G6PD screening and analysis of molecular G6PD mutations in Filipino and Laotian populations.