Eighteen patients hospitalized for excessive ovarian hyperstimulation syndrome are reported. In 14 cases the ovarian hyperstimulation was induced by human menopausal gonadotropins and in 4 cases by combined treatment with clomiphene and HCG. In 5 patients the hyperstimulation was associated with conception, which resulted in 1 quintuplet delivery, 1 early quintuplet abortion, 1 twin abortion, 1 normal delivery, and 1 missed abortion. The regimen of treatment was a conservative one. The patients were hospitalized and treated with infusion of plasma expanders. Anticoagulant therapy was administered only in cases that showed clinical evidence of thromboembolic phenomena or laboratory evidence of severe hemoconcentration. The pathogenesis of the ovarian hyperstimulation syndrome, prevention, and management are discussed. This syndrome should be diagnosed early and treated intensively.