Forty-five knees (thirty patients) with a specific diagnosis of synovial plica syndrome, and without any other known lesion, were randomized to be treated with either diagnostic arthroscopy alone or arthroscopy and division of all plicae. The diagnosis of synovial plica syndrome had been made on the basis of intermittent pain in the anterior aspect of the knee, painful clicking with activity, giving-way, and a palpable, tender plica. The patients were selected for arthroscopy only if the symptoms had continued unabated after a course of physical therapy. At the time of follow-up, improvement had occurred in only six (29 per cent) of the twenty-one knees in which the plicae had not been divided, in contrast with twenty (83 per cent) of the twenty-four knees in which they had been divided (p < 0.001). Ten (48 per cent) of the knees in which arthroscopic division had not been done were treated with another arthroscopic operation. Seven of these ten knees improved after the subsequent division of the plicae (p < 0.01). We concluded that synovial plicae of the knee may be a definite cause of anterior pain in children and adolescents.