The majority of cases of ornithine transcarbamylase deficiency are due to novel mutations making it impossible to develop common methods for genetic analysis. However, identification of causative mutations has important implications for diagnosis (particularly prenatal diagnosis), prediction of likely course and outcome and the eventual possibility of gene therapy. As part of a continuing study of ornithine transcarbamylase deficiency, we now report an additional thirty novel mutations in the ornithine transcarbamylase gene, together with a brief summary of their clinical presentations.