Gene therapy clinical trials raise important safety issues that complicate their design and require extensive preclinical testing. Human protocols for the treatment of arthritis and most other orthopaedic and rheumatologic indications are complicated additionally by the perception that they are largely acquired, nonlethal conditions. Taking these considerations into account, the first such human study used the local, ex vivo delivery of a gene whose product, the interleukin-1 receptor antagonist, has an outstanding safety profile. This gene was delivered to the metacarpophalangeal joints of postmenopausal women 1 week before these joints were removed during total joint replacement surgery. In addition to providing an additional safety cushion, the surgical removal of the genetically modified joints made available large amounts of tissue to examine for evidence of successful gene transfer and gene expression. This Phase I safety study was approved at the local and federal levels, and its funding was contingent on the establishment of an external monitoring board. This trial now has been completed and a Phase II, efficacy study is being planned. A similar study has begun in Dusseldorf, Germany and results from the first two patients are similar to the results of the American patients. Permission has been given for two additional human trials, one in the United States and one in the Netherlands, in which a gene encoding herpes thymidine kinase will be transferred to the joints of patients with rheumatoid arthritis who then will be administered gancyclovir. This procedure aims to treat the disease by producing a genetic synovectomy. Additional development of human gene therapies for arthritis and other orthopaedic and rheumatic conditions will be aided by the successful completion of these studies.