SLAP lesions disrupt the perilabral architecture, but so far there have been no reports about posterior instability due to SLAP lesions. In a prospective study of 30 patients with recurrent posterior instability, we found SLAP lesions as a reason for instability in three cases. The purpose of this study is to point out that SLAP lesions can be a cause of posterior instability. Thirty patients with clinical posterior shoulder instability underwent diagnostic arthroscopy before operative stabilization procedures, three of whom (three males, aged 29-51 years) showed a SLAP lesion (once case each of types II, III, and IV) as a cause of posterior instability. All three patients had a history of a fall on the outstretched arm. All patients underwent arthroscopic refixation of the labrum. After arthroscopic refixation of the SLAP lesions, two patients were completely stable (SLAP II and III), whereas one patient (SLAP IV) reported microinstability during overhead activity but complete stability during activity of daily living. The same patient complained about moderate pain in extreme external-flexion rotation with slightly reduced range of motion in external-flexion position. All other patients were free of pain and showed free range of motion. Our results demonstrate that SLAP lesions can be a cause for posterior shoulder instability. In our cases, posterior shoulder instability caused by SLAP lesions was successfully treated by arthroscopic refixation of the torn biceps anchor. When treating posterior shoulder instability, SLAP lesions should be taken into account.