A Madagascan child was followed for an homozygous sickle cell disease from the age of 7 to the age of 9 years. The clinical course of the disease, quite moderate till the age of 7, was then made more severe because of frequent painful crises during the observation period. Genetical and haematological data of the patient and his family showed that the two parents, heterozygote for sickle cell disease, and the homozygote patient, also had alpha-thalassemia. In the propositus patient, the association of the two disorders accounts for the quite moderate anaemia (haemoglobin level between 10 and 12 g/dl) and the persistent microcytosis. So, a microcytosis without iron deficiency and an haemoglobin level above 9 g/dl should suggest such an association.