Objective: To examine resolution of the diagnosis among parents of children with phenylketonuria (PKU) as a mechanism of adjustment for parents and children. Methods: Reaction to diagnosis interviews were conducted with 52 mothers and 47 fathers of 55 children with PKU aged 2-12 years. The parents also completed questionnaires assessing their personal adjustment (stress symptoms), their child' s adjustment (behavior problems), and coping variables (personal hopefulness and coping strategies). Results: Most mothers (69%) and fathers (77%) were resolved to their child' s diagnosis. Lower levels of parent stress were explained by higher personal hopefulness (14% of the variance for mothers and 21% for fathers) and resolution of the diagnosis (15% of the variance for mothers and 6% for fathers) after taking account of demographic variables and severity of the child' s PKU. Parent resolution, however, did not contribute independently to the variance explained in child behavior problems after taking account of coping variables and severity of PKU. Conclusions: Resolution of the diagnosis of PKU is a strong indicator of parent adjustment, and assessment of parent reactions should be considered an integral component of clinical care. Further research is warranted in relation to the implications of parent resolution for the child' s response to PKU through different development stages and the effectiveness of interventions in aiding parent resolution.