Gene therapy using adeno-associated virus (AAV) vectors is rapidly developing. AAV2 vectors are injected directly into the brain by stereotaxic brain surgery and have shown good results in Parkinson's disease and aromatic L-amino acid decarboxylase deficiency. An AAV9 vector-derived product for spinal muscular atrophy type 1 has been administered to more than 2000 patients worldwide. AAV vectors, which cross the blood-brain barrier, have been applied to the development of gene therapy for various neurological diseases. We are preparing for clinical trials for glucose transporter I deficiency, Niemann-Pick disease type C, GM2 gangliosidosis, sporadic amyotrophic lateral sclerosis, and spinocerebellar ataxia type 1. AAV.GT5, which can efficiently transfer genes to human hepatocytes, is planned for clinical trials for ornithine transcarbamylase deficiency and hemophilia B. Wildtype AAVs are considered to be nonpathogenic. The genome of the AAV vector resides in the episome and is rarely integrated into chromosomes, making the vectors highly safe. On the other hand, adverse events that require further attention are reported. Four patients died in a clinical trial of X-linked myotubular myopathy in which a high dose of AAV vector was administered. Clonal proliferation of hepatocytes was observed in a hemophilia canine long after gene therapy. Neuronal loss was observed in the dorsal root ganglia of primates treated with various AAV vectors.