Posterolateral rotatory instability of the elbow. Recurrent posterolateral rotatory instability of the elbow is an apparently undescribed clinical condition that is difficult to diagnose. We treated five patients, ranging in age from five to forty years, who had such a lesion and in whom the instability could be demonstrated only by what we call the posterolateral rotatory-instability test. This test involves supination of the forearm and application of a valgus moment and an axial compression force to the elbow while it is flexed from full extension. The elbow is reduced in full extension and must be subluxated as it is flexed in order to obtain a positive test result (a sudden reduction of the subluxation). Flexion of more than about 40 degrees produces a sudden palpable and visible reduction of the radiohumeral joint. The elbow does not subluxate without provocation. The cause for this condition, we think, is laxity of the ulnar part of the lateral collateral ligament, which allows a transient rotatory subluxation of the ulnohumeral joint and a secondary dislocation of the radiohumeral joint. The annular ligament remains intact, so the radio-ulnar joint does not dislocate. Operative repair of the lax ulnar part of the lateral collateral ligament eliminated the posterolateral rotatory instability, as revealed intraoperatively in our five patients. 