Familial hypercholesterolemia (FH) is an inherited disease of low density lipoprotein (LDL) receptor expression that has emerged as an important clinical model for the development of gene therapies directed to the liver. Patients who are homozygous for this disease have severe hypercholesterolemia and die of coronary artery disease in childhood. The success of orthotopic liver transplantation in correcting the underlying metabolic defect in FH confirms that the hepatocyte is an appropriate target for somatic gene transfer. The outcome of the first pilot study of liver-directed gene therapy follows. Five patients with homozygous familial hypercholesterolemia ranging in age from 7 to 41 years were enrolled; each patient tolerated the procedure well without significant complications. Transgene expression was detected in a limited number of hepatocytes of liver tissue harvested four months after gene transfer from all five patients. Significant and prolonged reductions in LDL cholesterol were demonstrated in three of five patients; in vivo LDL catabolism was increased 53% following gene therapy in a receptor negative patient, who realized a reduction in serum LDL equal to approximately 150 mg dl-1. This study demonstrates the feasibility of engrafting limited numbers of retrovirus-transduced hepatocytes without morbidity and achieving persistent gene expression lasting at least four months after gene therapy. The variable metabolic responses observed following low level genetic reconstitution in the five patients studied precludes a broader application of liver-directed gene therapy without modifications that consistently produce substantially greater gene transfer.