The majority of cases of microcytic anaemia (MCV < 80 fl) are caused by iron deficiency. However, ca. 10% of these have secondary anaemia, and on the other hand, ca. 30% of cases of iron deficiency anaemia have MCV > 80 fl (depending on the patient population). About 20% of menstruating women have iron deficiency anaemia at same stage of their reproductive age. Iron deficiency in women after reproductive age and in men is always abnormal. The cause behind significant iron deficiency is in these cases usually gastrointestinal bleeding . If the patient has microcytic anaemia (MCV < 80 fl) and secondary anaemia appears to be excluded, iron deficiency anaemia is likely. Secondary anaemias are unlikely if there is no relevant chronic disease and past history, clinical status, erythrocyte sedimentation rate, CRP, serum creatinine concentration or leucocytosis do not indicate the presence of an underlying disease. If the cause of iron deficiency is certain or evident, the situation is sufficiently well characterised and iron therapy may be initiated. Among some ethnic groups and especially in known families thalassaemias may be the primary causes of microcytic anaemia.