Background Obesity has been identified as the single most important risk factor for postoperative sternal infection in coronary bypass surgery patients. It is also a major risk factor for sternal dehiscence, with or without infection, for any type of cardiac operation. We assessed whether prophylactic measures could prevent this complication. Methods Two studies were conducted. In study A, 3,158 heart surgery patients were analyzed at 3 cardiac units. Obesity was defined as body mass index (BMI) more than 30. Group I (1,253 obese [39.7%]) was compared with group II (1,905 nonobese [60.3%]). Sternal closure was done at the surgeon's preference: (a) plain wires through and through the bone; (b) peristernal figure-of-eight wires; or (c) peristernal method, using stainless-steel cables. In study B, 123 obese patients were prospectively divided into 2 subgroups. Group B-1 (54 patients) underwent lateral prophylactic sternal reinforcement before placement of peristernal wires. Group B-2 (69 patients) had standard sternal closure, as in study A. Results In study A, group I had 81 dehiscences (6.46%); 78 also suffered deep sternal infection and mediastinitis (96%). Despite treatment, dehiscence recurred in 13, and mortality was 38.4%. In group II nonobese patients, 31 dehisced (1.6%, p = 0.000), with no mortality. In study B, group B-1 (54) had 0% dehiscence versus group B-2 (69) with 6 dehiscences (8.7%). Conclusions In our study, the rate of obesity is high (∼ 40%). Sternal dehiscence is real when the BMI is more than 30 (6.46%), and has high morbidity and mortality. Prophylactic sternal reinforcement seems to prevent this complication.