Objective: To determine the prevalence of thalassemia including α-thalassemia-1 trait (SEA type), β-thalassemia trait, hemoglobin E (HbE) trait, homozygous HbE, the combination of α-thalassemia-1 (SEA type) and β-thalassemia trait, α-thalassemia-1 (SEA type) and hemoglobin E trait, and β-thalassemia hemoglobin E disease in pregnant women. Method: A cross-sectional descriptive study was conducted on pregnant women who attended the antenatal clinic at Maharaj Nakorn Chiang Mai Hospital, from 1 August to 31 October 2001. All subjects had blood taken for diagnosis of thalassemia trait or diseases, based on quantitative electrophoresis, and PCR (polymerase chain reaction) technique Results: 516 pregnant women were recruited. 81.0% resided in Chiang Mai province, and the remainder were in other northern provinces of Thailand. The mean (± SD) age was 27.7 ± 6.3 years old. 5.6% of cases had anemia. Overall prevalence of thalassemia trait was 25.4% which were classified as follows: α-thalassemia-1 (SEA type) trait 6.6%, β-thalassemia trait 3.7%, hemoglobin E trait 11.6%, homozygous hemoglobin E 0.8%, the combination of α-thalassemia-1 (SEA type) and β-thalassemia trait 1.2% and the combination of α-thalassemia-1 (SEA type) and hemoglobin E trait 1.5%. Additionally, the authors also found β-thalassemia hemoglobin/E disease 0.2%. Conclusion: The prevalence of thalassemia carriers among pregnant women at Maharaj Nakorn Chiang Mai hospital was high, indicating the necessity of a screening thalassemia program aimed at prevention and control of this disease.