Maternal weight gain and intake of selected nutrients were correlated with plasma phenylalanine (Phe) concentrations and reproductive outcomes (in 150 and 142 subjects respectively) in the Maternal PKU Collaborative Study. Daily protein intake was negatively correlated with plasma Phe concentration. Birth length, weight and head circumference of offspring were negatively influenced by the length of time required for the maternal plasma Phe to decline below 600 mumol/l (10 mg/dl) and positively influenced by weight gain of mother as a percentage of recommended weight gain. Birth weight and length were positively correlated with maternal protein and energy intakes. During the first trimester, intakes of fat, calcium, phosphorus, vitamin A and folate were significantly greater in women who had a good reproductive outcome than by women who had a poor outcome. In addition to plasma Phe control in maternal phenylketonuria (PKU), maternal weight gain and dietary intake of protein, energy and fat were correlated with outcome. Therefore, nutrient intake and maternal weight gain should be considered along with plasma Phe concentration when managing the therapy of a pregnant woman with PKU.