The annual incidence of aplastic anemia in metropolitan Bangkok, Thailand, and its five suburban provinces was prospectively determined. All patients first diagnosed during the period from January through December 1989 who met specific clinical and pathologic criteria were included. Thirty-two cases were identified, yielding an overall incidence of 3.7 per million. The incidence rates for the age groups 0 through 24, 25 through 59, and over 60 years were 4.3, 3.2, and 2.1 per million, respectively; the highest rate, 7.2 per million, was found for individuals aged 15 to 24 years. The male-to-female ratio was 1.9. The incidence of aplastic anemia in Bangkok is higher than that reported in recent European studies. The peak rate in young persons is almost fourfold higher than in comparable recent western studies and suggests an environmental etiology peculiar to Thailand.