Mild forms of alpha thalassaemia are difficult to detect in adults. Since alpha thalassaemia existing with structural defects of the beta chain of haemoglobin may lead to decreased levels of the abnormal haemoglobin, we examined individuals having sickle-cell trait for the possible coexistence of alpha thalassaemia. Patients with sickle-cell trait having haemoglobin-S (Hb-S) levels less than commonly expected were compared to two control groups--one with sickle-cell trait and the usual levels of Hg S and one with normal haemoglobin. Twenty-one patients with sickle-cell trait having Hb-S concentrations below 35% had 65.8% Hb A, 31.8% Hb S, and a mean corpuscular volume of 81.6 fl. Studies of the relative rates of alpha-, betaS- and betaA-chain synthesis in 14 of these individuals showed a mean alpha:beta ratio of 0.76. In patients with normal haemoglobin as well as in sickle-cell trait with Hb-S levels above 35%, this ratio was unity. These findings are consistent with the presence of alpha thalassaemia in patients with sickle-cell trait who have lower than usual levels of Hb S and microcytosis.