Medial synovial shelf plica syndrome is caused by acquired thickening and inflammation of a commonly present residual embryonic synovial fold. Treatment with a local injection into the plica and surrounding synovium of a steroid and a long-acting local anesthetic was tested in a series of 30 patients to see if more involved and expensive treatment could be avoided. Thirty-one knees were studied and treated prospectively; rigid criteria for making the diagnosis were maintained. Twenty-two (73%) patients had complete relief of pain and full return to activity; five patients had some amelioration of their symptoms and partial return to activity; and three patients had poor results. Two of the poor results were secondary to errors in diagnosis, and one was secondary to the presence of mature fibrosis confirmed during subsequent arthroscopic resection. As a control, ten patients were injected with long-acting local anesthetic alone. In all ten, symptoms were relieved only for the duration of the anesthetic. Intraplical steroid injection appears to be a reasonable, prudent, initial step in the treatment of medial synovial shelf plica syndrome. For competitive athletes, it provides very short morbidity and the ability to return to full practice and participation in a very short period of time.