It has been suggested that early development of the incisional hernia is caused by perioperative factors, such as surgical technique and wound infection. Late development may implicate other factors, such as connective tissue disorders. Our objective was to establish whether incisional hernia develops early after abdominal surgery (i.e., during the first postoperative month). Patients who underwent a midline laparotomy between 1995 and 2001 and had had a computed tomography (CT) scan of the abdomen during the first postoperative month were identified retrospectively. The distance between the two rectus abdominis muscles was measured on these CT scans, after which several parameters were calculated to predict incisional hernia development. Hernia development was established clinically through chart review or, if the chart review was inconclusive, by an outpatient clinic visit. The average and maximum distances between the left and right rectus abdominis muscles were significantly larger in patients with subsequent incisional hernia development than in those without an incisional hernia (P < 0.0001). Altogether, 92% (23/25) of incisional hernia patients had a maximum distance of more than 25 mm compared to only 18% (5/28) of patients without an incisional hernia (P < 0.0001). Incisional hernia occurrence can thus be predicted by measuring the distance between the rectus abdominis muscles on a postoperative CT scan. Although an incisional hernia develops within weeks of surgery, its clinical manifestation may take years. Our results indicate perioperative factors as the main cause of incisional hernias. Therefore, incisional hernia prevention should focus on perioperative factors.