BACKGROUND: Inversion injuries, primarily sprains, of the ankle are one of the most commonly treated injuries. The three main treatment modalities for acute lateral ankle ligament injuries are immobilisation with plaster cast or splint, 'functional treatment' comprising early mobilisation and the use of an external support (e.g. ankle brace or taping), and surgical repair or reconstruction. OBJECTIVES: We aimed to compare surgical versus conservative treatment for acute injuries of the lateral ligament complex of the ankle in adults. SEARCH STRATEGY: We searched the Cochrane Musculoskeletal Injuries Group specialised register (December 2001), the Cochrane Controlled Trials Register (The Cochrane Library, Issue 4, 2001), MEDLINE (1966 to May 2000), EMBASE (1980 to May 2000), CURRENT CONTENTS (1993 to 1999), BIOSIS (to 1999), reference lists of articles, and contacted organisations and researchers in the field. SELECTION CRITERIA: Randomised or quasi-randomised controlled trials comparing surgical with conservative interventions for treating acute injuries to the lateral ligament complex of the ankle in adults. DATA COLLECTION AND ANALYSIS: At least two, usually three, reviewers independently assessed methodological quality and extracted data. Where appropriate, results of comparable studies were pooled using fixed and random effects models and relative risks with 95% confidence intervals for dichotomous outcomes calculated. Heterogeneity between trials was tested using a standard chi-squared test. We undertook sensitivity analyses to explore the effects of inclusion of quasi-randomised trials and subgroup analyses by the two main methods of conservative treatment. MAIN RESULTS: Seventeen studies, involving a total of 1950 mostly young active adult males, were included. All trials had methodological flaws. In particular, allocation was rated as effectively concealed in only one trial and there was no report of masking outcome assessors to the interventions in any trial. Outcome measures were variously defined and data for pooling for individual outcomes were only available for a maximum of 11 studies. While there were statistically significant differences for three primary outcomes (non-return to pre-injury level of sports; undefined pain or pain on activity; and subjective or functional instability) in favour of the surgical treatment group using the fixed effects model, these findings failed to demonstrate robustness in sensitivity analyses. Exclusion of the data from explicitly quasi-randomised trials or, given the highly statistically significant heterogeneity, analyses using the random effects model modified these findings to non-statistically significant differences. There was no statistically significant difference in ankle sprain recurrence, the other primary outcome measure. Though there was a statistically significantly higher incidence of objective instability in conservatively treated patients, the functional implications of this are uncertain. There was some evidence for a lower incidence of long-term ankle swelling in surgically treated patients. However, as well as tending to take longer to resume normal activities, including work, there was some limited evidence from a few trials for a higher incidence of ankle stiffness, impaired ankle mobility and complications in the surgical treatment group. Subgroup analyses by type of conservative treatment (plaster cast or functional treatment) revealed no statistically significant differences in effect for any of the six outcomes examined. REVIEWER'S CONCLUSIONS: There is insufficient evidence available from randomised controlled trials to determine the relative effectiveness of surgical and conservative treatment for acute injuries of the lateral ligament complex of the ankle. Sufficiently powered, good quality and adequately reported randomised trials of primary surgical repair versus the best available conservative treatment for well-defined injuries are required.