Human gene therapy trials directed at hereditary diseases, including adenosine deaminase (ADA) deficiency, familial hypercholesterolemia, and cystic fibrosis, are reviewed. Human gene therapy involves the introduction and expression of recombinant genes in somatic, nonreproductive cells with the intent to reverse or prevent a particular disease. Two methods for introducing genes into human cells are currently being used in clinical trials. Ex vivo gene delivery involves removing targeted cells from the patient's body, introducing the recombinant gene into the cells, and placing the modified cells back into the patient's body. In vivo gene delivery involves placing the recombinant gene directly into the patient's body, targeting the tissue or cell of interest. The transfer of the recombinant gene into the cell and the subsequent expression of the transgene product are the rate-limiting steps for successful gene therapy. A variety of methods, including the use of modified viruses and synthetic vectors, are currently being used in clinical trials. Since the approval and initiation of the first human gene therapy trial to treat ADA deficiency in 1989, there have been more than 170 approved gene therapy trials in the United States. More than 1500 patients have been enrolled in human gene therapy trials worldwide. Preliminary clinical trials have targeted diseases such as ADA deficiency, familial hypercholesterolemia, and cystic fibrosis. These trials have employed variable designs and strategies, making interpretation of the results difficult. However, the initial data are encouraging, and the procedures have been well tolerated. The clinical utility of human gene therapy remains to be defined; immediate efforts will focus on improving vector design to limit toxicity and enhance the efficiency of gene transfer.