The haematological indices of Peninsular Arabs (United Arab Emirate Nationals, Yemeni and Omani) have been examined. The most outstanding feature, seen in 40-50% of all subjects, was one of hypochromia, microcytosis associated with erythrocytosis. In approximately 5% the hypochromia was severe (MCH 19-22 pg) and 20% of these were found to have beta thalassaemia trait. In 10% of subjects the hypochromia was moderate (MCH 23-24 pg) and beta thalassaemia was confirmed in only 10%. The remaining 25% had a mild hypochromia (MCH 25-27 pg) and no beta thalassaemia was detected. The cause of the hypochromia in subjects with a normal Hb A2 (30% of the total population) is probably alpha thalassaemia, firstly because in those patients with an MCH of 19-24 pg the other haematological parameters are statistically the same as those with proven beta thalassaemia and, secondly, in those with an MCH of 25-27 pg iron deficiency is not common (6% of the population). The degree and pattern of the distribution of hypochromia of the three major ethnic groups of the Peninsular Arabs could be explained either by different alpha and beta thalassaemia genes being operative or by different degrees of inbreeding of the same genes.