Hb Mizuho, an unstable beta chain variant with a Leu----Pro substitution at position beta 68, was observed in a young Caucasian boy from Kentucky. Identification was made by sequence analyses of amplified DNA and by hybridization of amplified DNA with specific probes. The patient had high Hb F levels up to the present age of nearly 5 years; this high Hb F might in part be responsible for his condition which was considerably milder than that seen in the two patients described in earlier reports. The increased gamma chain synthesis may be due to special characteristics of the beta-globin gene cistron which are comparable to those observed in sickle cell anemia patients with a relatively mild disease.