Ankle sprains have a high incidence of associated injuries and conditions that may be unrecognized at the initial time of injury. Failure to treat these conditions at the index surgery may compromise outcomes and delay recovery. The purpose of this study was to determine the type and frequency of associated injuries and conditions in military patients with chronic lateral ankle instability. Between 1996 and 2002, 160 patients had 180 modified Broström-Gould lateral ankle ligament reconstructions for chronic ankle instability. A retrospective review of the clinical history, physical examination, radiographs, and intraoperative findings was conducted. The overall incidence of associated extra-articular conditions and injuries found in this study was 64%; 115 conditions were identified in 180 ankles. Peroneal tendon injuries occurred with the highest frequency (28%), followed by os trigonum lesions (13%), lateral gutter ossicles (10%), hindfoot varus alignment (8%), anterior tibial spurs (3%), and tarsal coalitions (2%). Twenty revision lateral ankle ligament reconstructions were required for either persistent pain or recurrent instability. The most common associated conditions were undiagnosed hindfoot varus alignment abnormalities (28%) followed by untreated peroneal injuries (25%). This study confirms the frequency of conditions associated with lateral ankle instability and emphasizes several conditions that have received little attention in the literature. Identifying these associated conditions before surgery enables the surgeon to treat all conditions at one operation, returning the patient to full activity sooner. Guidelines are presented to assist clinicians in screening patients for these associated conditions.