The clinical, hematological, and biochemical characteristics of a new family with heterozygous hemoglobin (Hb) Louisville are described. The family showed a decrease in both oxygen affinity and cooperativity with the normal Bohr effect. This family has the greatest number of affected members reported to date. Among the descendants, two first cousins (III-10 and III-11), both of whom are affected by the heterozygous trait of Hb Louisville, had had three abortions of undetermined causes.