The unstable hemoglobins Istanbul and Saint-Etienne have the same amino acid substitution (α2β2 92F8 His→Gln). Despite this, there are some clinical and hematological differences between the individual with Hb Istanbul and the one with Hb Saint-Etienne. These are: (1) the clinical course of the patient with Hb Istanbul before splenectomy was more severe when compared with that of the individual with Hb Saint-Etienne; (2) although Hb F was found within normal range in the individual with Hb Istanbul, a high level of Hb F (19%) was observed in the patient with Hb Saint-Etienne. The alkali-resistant Hb found in the former was of adult type and in the latter case it was of fetal type. The possible explanations for these differences are discussed.