Strict control of hyperphenylalaninemia is necessary in pregnant women with phenylketonuria (PKU) in order to prevent phenylalanine embryopathy in the fetus, characterized by intrauterine growth restriction, dysmorphic facies, congenital heart disease, microcephaly and intellectual disability, collectively known as maternal PKU syndrome. Sapropterin dihydrochloride (SD), an alternative or adjunct to dietary therapy in patients with tetrahydrobiopterin (BH4)-responsive PKU, has recently been used in several cases to treat PKU during pregnancy with satisfactory results. Here, we report two pregnancies treated with SD and unrestricted diet in a patient with BH4-responsive mild PKU. The first pregnancy resulted in a partial hydatidiform mole and was terminated, whereas a healthy infant was born from the second pregnancy. Phenylalanine control was optimal in both pregnancies. To the best of our knowledge, this is the first report on the development of partial hydatidiform mole associated with SD treatment and the second report on molar pregnancy in PKU. While the relation between SD and molar pregnancy is unknown, further studies may be needed to investigate the possible effects of SD on fertilization.