SummaryDiagnosis of beta-thalasscmia minor was evoked among 9 Belgian individuals from 6 nonrelated families on the basis of microcytosis (mean MCV 67μ.3; range: 59–74μ1) and confirmed by determination of Hb A2 levels (mean 5,4%; range 3.9 to 7.1 %). Iron deficiency, another common cause of microcytosis, was excluded in 6/7 subjects. These individuals do not have clinical manifestations needing hospitalization. It has been estimated that in the Brussels region birth of one child with betathalasscmia major may be expected each year and that the cost resulting from detection programmes of carrier states would be lower than the cost for treatment of beta-thalassemia major. In view of the high numbers of immigrants in Belgium and to avoid intcmpestive iron treatment, detection of beta-thalasscmia minor must also be performed among Belgian individuals presenting with microcytosis. Two major hypotheses are proposed for explaining the presence of bcta-thalassemic gene among North European populations: spontaneous mutations or persistence of old mutations introduced during successive invasions of this country.