Eighty-five patients who attended at Ramathibodi Hospital during November 1994 to June 1996 were investigated for thalassemia genotype, hemoglobin (Hb) typing and blood cell parameters. All patients were screened primarily for complete blood count using the Technicon H*3 automated hematology analyzer and Hb typing using the automated HPLC. Their genotypes were evaluated by in vitro gene amplification using primers for detection of common alpha-thalassemic genes found in the Thai population. We found 45 cases out of 85 were alpha-thalassemia trait with A2A typing, 10 were normal, 7 were alpha-thalassemia trait complicated by beta-thalassemia trait or HbE trait, 18 were HbH disease or HbH with Hb Constant Spring (HbH/CS), and 5 were AE Bart's disease. The alpha-thalassemia 1 trait had heterogeneity in red cell population as shown by increased red cell distribution width (RDW), the increased percent microcytic red cell (%Micro) and decreased mean cell volume (MCV). Red cell parameters in alpha-thalassemia 2 trait and HbCS trait were not significantly different from normal. The cases with coinheritance of alpha-thalassemia trait with beta-thalassemia trait or with HbE trait showed variation in their red cell parameters: one case showed less abnormal red cell parameters than those of uncomplicated alpha-thalassemia but the other two cases showed unimproved values. The homozygous alpha-thalassemia 2 showed similar red cell parameters to the alpha-thalassemia 1 trait. In conclusion, we can screen the alpha-thalassemia 1 trait and homozygous alpha-thalassemia 2 by using the simple red cell parameters such as the MCV and RDW; however, they must be confirmed for alpha-thalassemic genes. Unfortunately, red cell parameters of alpha-thalassemia 2 trait or HbCS trait were not different from those of normal subjects.