OBJECTIVE: To estimate the disease burden of the most important complications of postoperative abdominal adhesions: small bowel obstruction, difficulties at reoperation, infertility, and chronic pain. DESIGN: Systematic review and meta-analyses. DATA SOURCES: Searches of PubMed, Embase, and Central, from January 1990 to December 2012, without restrictions to publication status or language. STUDY SELECTION: All types of studies reporting on the incidence of adhesion related complications were considered. DATA EXTRACTION AND ANALYSIS: The primary outcome was the incidence of adhesive small bowel obstruction in patients with a history of abdominal surgery. Secondary outcomes were the incidence of small bowel obstruction by any cause, difference in operative time, enterotomy during adhesiolysis, and pregnancy rate after abdominal surgery. Subgroup and sensitivity analyses were done to study the robustness of the results. A random effects model was used to account for heterogeneity between studies. RESULTS: We identified 196 eligible papers. Heterogeneity was considerable for almost all meta-analyses. The origin of heterogeneity could not be explained by study design, study quality, publication date, anatomical site of operation, or operative technique. The incidence of small bowel obstruction by any cause after abdominal surgery was 9% (95% confidence interval 7% to 10%; I(2)=99%). the incidence of adhesive small bowel obstruction was 2% (2% to 3%; I(2)=93%); presence of adhesions was generally confirmed by emergent reoperation. In patients with a known cause of small bowel obstruction, adhesions were the single most common cause (56%, 49% to 64%; I(2)=96%). Operative time was prolonged by 15 minutes (95% confidence interval 9.3 to 21.1 minutes; I(2)=85%) in patients with previous surgery. Use of adhesiolysis resulted in a 6% (4% to 8%; I(2)=89%) incidence of iatrogenic bowel injury. The pregnancy rate after colorectal surgery in patients with inflammatory bowel disease was 50% (37% to 63%; I(2)=94%), which was significantly lower than the pregnancy rate in medically treated patients (82%, 70% to 94%; I(2)=97%). CONCLUSIONS: This review provides detailed and systematically analysed knowledge of the disease burden of adhesions. Complications of postoperative adhesion formation are frequent, have a large negative effect on patients' health, and increase workload in clinical practice. The quantitative effects should be interpreted with caution owing to large heterogeneity. REGISTRATION: The review protocol was registered through PROSPERO (CRD42012003180).