The purpose of this study was to evaluate the long-term results of arthroscopic resection of a medial plica and to describe the usefulness of the clinical findings and MRI for preoperative diagnostics. From the baseline population of 172,777 military conscripts, thirty-three consecutive young adult patients with normal preoperative MRIs of the knee and a sole postoperative diagnosis of medial plica were treated with arthroscopic plica resection. Functional outcome was evaluated at a final follow-up in 25 patients with 34 knees with Kujala, Lysholm and visual analog scale (VAS) scores. Functional results were excellent to good in 17 patients, fair in three patients, and poor in 3 patients. The median Kujala score was 92 (25-100), the median Lysholm score 89 (26-100), and the median VAS 1.4 (0-8.8). Median follow-up time was 6.6 years (3.6-8.7 years). Most patients had no history of direct knee trauma preceding the symptoms. No statistically significant correlation was seen between MRI classification of the plica size or clinical findings compared to arthroscopic classification. Resection of the medial plica in a symptomatic knee has good to excellent functional long-term outcome in the majority of cases, and the procedure is not associated with postoperative complications. MRI and preoperative clinical examination seem to be unreliable in detecting medial plicae.