Maternal phenylketonuria (MPKU) is an increasingly common concern for health professionals involved in prenatal care. This single-participant study investigated the efficacy of prenatal psychotherapy in improving metabolic control, an important consideration in MPKU pregnancies. Results indicated a robust and positive relationship between psychotherapy and metabolic control. Levels of negative mood and levels of phenylalanine control were also strongly and positively correlated. Implications for treating MPKU are discussed, and prenatal psychotherapy is forwarded as a means of improving birth outcomes in this population.