Anemia is a major public health problem in Thailand with the estimate prevalence of 25 per cent. The last survey of the prevalence of anemia was done in 1979, so we performed another survey to observe the trend of change in prevalence of anemia in Khon Kaen. We conducted a community survey by multistage random sampling. In a population aged 30-65 years in 13 villages in Amphoe Phon. The mean hematocrit level was 40.1 +/- 5.1 per cent (42.8 +/- 4.6% in males, 38.1 +/- 4.5% in females). The prevalence of anemia (Hct < 39% in males, < 36% in females) was 22.2 per cent (16.4% in males, 26.4% in females). The mean level of hematocrit of patients admitted for elective surgery in Srinagarind Hospital was 38.6 +/- 5.5 per cent which was significantly lower than in the rural area (P < 0.001) and the prevalence of anemia was 25.9 per cent. The prevalence of anemia increased with age and decreased with high education and body mass index. Comparing to the data in 1979, the prevalence in males increased from 13.9 to 16.4 per cent and from 23.3 to 26.4 per cent in females. The prevalence of anemia in Khon Kaen is still high and has increased compared to the data in 1979, and thus, remains a major public health problem.