Background: Bariatric surgery carries a risk of severe postoperative complications, sometimes leading to reinterventions or even death. The incidence and risk factors for reintervention and death within 90 days after bariatric surgery are unclear, and were examined in this study. Methods: This population-based cohort study included all patients who underwent bariatric surgery in one of the five Nordic countries between 1980 and 2012. Data on surgical and endoscopic procedures, diagnoses and mortality were retrieved from national high-quality and complete registries. Multivariable Cox regression analysis was used to calculate hazard ratios (HRs), adjusted for country, age, sex, co-morbidity, type of surgery and approach, year and hospital volume of bariatric surgery. Results: Of 49 977 patients, 1111 (2·2 per cent) had a reintervention and 95 (0·2 per cent) died within 90 days of bariatric surgery. Risk factors for the composite outcome reintervention/mortality were older age (HR 1·65, 95 per cent c.i. 1·36 to 2·01, for age at least 50 years versus less than 30 years) and co-morbidity (HR 2·66, 1·53 to 4·62, for Charlson co-morbidity index score 2 or more versus 0). The risk of reintervention/mortality was decreased for vertical banded gastroplasty compared with gastric bypass (HR 0·37, 0·28 to 0·48) and more recent surgery (HR 0·51, 0·39 to 0·67, for procedures undertaken in 2010 or later versus before 2000). Sex, surgical approach (laparoscopic versus open) and hospital volume did not influence risk of reintervention/mortality, but laparoscopic surgery was associated with a lower risk of 90-day mortality (HR 0·29, 0·16 to 0·53). Conclusion: Reintervention and death were uncommon events within 90 days of bariatric surgery even in this unselected nationwide cohort from five countries. Older patients with co-morbidities have an increased relative risk of these outcomes.