This report describes the findings of a screening program of 918 obstetric patients for thalassemia minor. Patients with erythrocyte mean corpuscular volume (MCV) less than 80 fL on initial complete blood count were evaluated with serum iron, total iron binding capacity, quantitative hemoglobin electrophoresis, and trial of iron replacement. A diagnosis of thalassemia minor was made when microcytosis persisted after exclusion of iron deficiency or other causes of erythrocyte microcytosis. Twenty-six women (2.8% of those screened) had an initial MCV less than 80 fL. Three cases of previously unsuspected thalassemia minor were detected (one alpha-thalassemia, two beta-thalassemia). Of 17 well-documented cases of iron deficiency, 16 had a hemoglobin level above 11 g/dL on initial complete blood count and would not have been otherwise detected until much later in gestation. A simple screening program can effectively identify pregnant women with unrecognized thalassemia minor and can also detect patients with iron deficiency before they become anemic.