The rare hemoglobinopathies with abnormal oxygen binding are usually characterized by erythropoietin-mediated erythrocytosis. Bare et al. first described a hemoglobinopathy with mild erythrocytosis in a 22-year-old Caucasian woman in 1976. These authors called the abnormal hemoglobin Hb York. Hb York is characterized by a mutation at the beta146 position that changes histidine into proline. A second case of Hb York was observed by Kosugi et al. in 1983. To the best of our knowledge, no further cases have been reported. We have encountered a new case of Hb York, which was detected by agar gel electrophoresis at pH 6.0. Analysis of DNA sequences revealed a CAC-->CCC mutation in codon 146. The proportion of Hb York was approximately 50%. Analysis of oxygen transport function showed a leftward shift of the sigmoidal O2-dissociation curve. P50 was reduced to 15.5 mmHg. Investigation of family members revealed Hb York in the patient's sister, two daughters and a grandson. In retrospect, the mother of the patient may also have been affected. The mode of inheritance is autosomal dominant.