Introduction of dystrophin by gene transfer into the dystrophic muscles of Duchenne muscular dystrophy (DMD) patients has the possibility of triggering an immune response as many patients will not have been exposed to some (or all) of the epitopes of dystrophin. This could in turn lead to cytotoxic destruction of transfected muscle fibres. We assessed such concerns in the dystrophin-deficient mdx mouse using plasmid DNA as the gene transfer system. This avoids complications associated with the administration of viral proteins. Gene transfer of cDNAs encoding mouse full-length or a truncated minidystrophin did not evoke either a humoral or cytotoxic immune response. Mdx mice may be tolerant due to the presence of rare 'revertant' dystrophin-positive fibres in their skeletal muscles. In contrast, gene transfer of human full-length or minidystrophin provoked both humoral and cytotoxic responses leading to destruction of the transfected fibres. These experiments demonstrate the potential risk of deleterious effects following gene therapy in DMD patients and lead us to suggest that patients enrolled in gene therapy trials should ideally have small, preferably point, mutations and evidence of 'revertant' dystrophin-positive muscle fibres.