Several members representing three generations of two families with erythrocytosis due to a newly described hemoglobinopathy (Hb Linköping, beta 36 Pro----Thr) were studied. All affected family members had a pronounced left-shift of the oxygen dissociation curve, indicating an increased oxygen affinity leading to compensatory erythrocytosis. The proband of each family was found by a simple method determining the oxygen pressure at half hemoglobin saturation (P50), which was used as a screening test evaluating patients with polycythemia. In both cases the hemoglobinopathy was missed by routine hemoglobin electrophoresis. The abnormal hemoglobin was confirmed by isoelectric focusing and the altered amino acid sequence was identified by reversed-phase high-performance liquid chromatography (HPLC). The importance of a correct diagnosis in patients with erythrocytosis due to high-affinity hemoglobin variants is discussed.