Purpose To estimate the incidence of and risk factors for stoma site hernia after closure of a temporary diverting ileostomy. Method In a non-comparative cohort study, charts (n = 216) and CT-scans (n = 169) from patients who had undergone loop ileostomy closure following low anterior resection for rectal cancer 2010-2015 (mainly open surgery) at three hospitals were evaluated retrospectively. Patients without hernia diagnosis were evaluated cross-sectionally through a questionnaire (n = 158), and patients with symptoms of bulging or pain were contacted and offered a clinical examination or a CT scan including Valsalva maneuver. Results In the chart review, five (2.3%) patients had a diagnosis of incisional hernia at the previous stoma site after 8 months (median). In 12 patients, the CT scan showed a hernia, of which 8 had not been detected previously. The questionnaire was returned by 130 (82%) patients, of which 31% had symptoms of bulging or pain. Less than one in five of patients who reported bulging were diagnosed with hernia, but the absolute majority of the radiologically diagnosed hernias reported symptoms. By combining clinical and radiological diagnosis, the cumulative incidence of hernia was 7.4% during a median follow up time of 30 months. Risk factors for stoma site hernia were male sex and higher BMI. Conclusion Hernia at the previous stoma site was underdiagnosed. Less than a third of symptomatic patients had a hernia diagnosis in routine follow up. Randomized studies are needed to evaluate if prophylactic mesh can be used to prevent hernias, especially in patients with risk factors.