A short multiple-choice knowledge test on maternal phenylketonuria was developed and validated. It was administered to 49 young female patients participating in a longitudinal study. The test was analyzed for its psychometric qualities and was found to be reliable: Cronbach alpha 0.62. Its validity was indicated by a moderate correlation with IQ scores (r = 0.40) and by its significantly differentiating between subjects who participated in patient education group meetings and those who did not. Items varied in difficulty, indicating which areas of knowledge need special educational efforts. The test was recommended for use to assess patients' knowledge, to identify misconceptions that appear to be remedied by patient education, to stimulate group discussions and to help evaluate educational programs.