Patients with anterior cruciate ligament (ACL) injury experience atrophy and weakening of the extensor as well as the hamstrings muscles at the injured leg. Especially, the weakness of the quadriceps muscle has been ascribed to hamper daily physical tasks. The purpose of the present study was therefore to investigate if nutrient supplementation during 12 weeks of conservative rehabilitation strength training could enhance hypertrophy and strength of the quadriceps muscle in ACL-injured patients. Twenty-six ACL-injured men and women were included and randomly distributed into three supplementation groups: Protein+Carbohydrate (PC), Isocaloric-Carbohydrate (IC), or Placebo (PL), ingesting the supplementation immediately after each of 36 training sessions. Determined from images of thigh cross-sections (magnetic resonance imaging) the hypertrophy of the quadriceps muscle differed significantly between groups at the distal part, with the PC group demonstrating the largest hypertrophy. Peak torque of the quadriceps muscle at constant velocity 60 degrees.s-1 was significantly elevated in the PC group only, and the time to reach peak torque tended to decrease as well only in the PC group. The results from this study demonstrate that restoration of the distal vasti muscle mass and knee extension muscle strength with resistance training is promoted further by protein-containing nutrient supplementation immediately after single exercise sessions. Thus, exercise-related protein supplementation may seem important for surgery-related rehabilitation of skeletal muscle. Copyright (c) 2006 Orthopaedic Research Society.