In a separate 10-week, placebo-controlled study, 45 PKU patients with blood phenylalanine levels controlled on a stable phenylalanine-restricted diet (blood phenylalanine ≤ 480 μ mol/ l on enrolment) were randomized 3:1 to treatment with sapropterin dihydrochloride 20 mg/ kg/ day (n=33) or placebo (n=12).