Hemoglobin (Hb) SD-Los Angeles compound heterozygotes usually have a severe clinical course although the effect of an elevated Hb F on the clinical phenotype has not been reported previously. We describe 5 Kuwaiti children with Hb SD associated with Hb F levels >20%. They all presented with sickling-related symptoms by ≤2 years of age and have been followed for 3–15 years. All had severe clinical courses marked by varying degrees of splenic sequestration crises, acute chest syndrome, vaso-occlusive crises, osteomyelitis and avascular necrosis of the femoral head. This pattern is in contrast with the usually mild presentation in Kuwaiti Hb SS patients with elevated Hb F. It therefore appears that Hb F does not ameliorate the clinical phenotype in Hb SD-Los Angeles. The reasons for this are not quite clear.