This study aimed to evaluate systematically the effect of the free use of fruits and vegetables containing an intermediate amount of phenylalanine (51-100 mg/100 g) on the biochemical control in children with phenylketonuria (PKU). Fifteen subjects with PKU, with a median age of 6 years (range 1-24 years) were studied. In a three-part prospective 15-week study, subjects sequentially ate fruits and vegetables containing phenylalanine 0-50 mg/100 g for weeks 1 to 3; 51-75 mg/100 g for weeks 4 to 8; and 76-100 mg/100 g for weeks 9 to 15. Plasma phenylalanine concentrations were measured twice daily for three consecutive days in weeks 1, 3, 6, 8, 11, 13 and 15. A standard menu was followed on the blood sampling days. Daily dietary records of fruits and vegetables under study were kept throughout the trial. Control of phenylalanine concentrations was not adversely affected by the free use of fruits and vegetables containing 51-100 mg/100 g. Pre-breakfast median plasma concentrations were: weeks 1 to 3, 260 micromol/L (range 90-890); weeks 4 to 8, 255 micromol/L (range 130-920); and weeks 9 to 15, 278 micromol/L (range 30-880). Pre-evening meal median plasma phenylalanine concentrations were: weeks 1 to 3, 240 micromol/L (range 30-820); weeks 4 to 8, 210 micromol/L (40-880); and weeks 9 to 15, 238 micromol/L (range 20-880). These data suggest that free use of fruits and vegetables containing 51-75 mg/100 g poses no problem for children with PKU.