Objectives: Knowledge of the natural course of acute pancreatitis (AP) and risk of progression to chronic pancreatitis (CP) is limited. The aims were to describe: (1) the incidence of progression from AP to CP, (2) prognostic factors for progression, and (3) the natural course and mortality of progressive AP. Methods: During 1977 to 1982, patients admitted to hospitals in Copenhagen with a diagnosis of AP or CP were included in a prospective cohort and followed up by the Danish registries in 2008. The subcohort analyses comprised 352 AP patients. Results: Progressive AP was found in 85 patients (24.1%) during follow-up; 48.2% developed from alcoholic AP, 47.0% from idiopathic AP, and 4.8% from other causes. The mortality rate for patients with progressive AP was 2.7 times higher than in patients with nonprogressive acute pancreatitis, and 5.3 to 6.5 times higher than in the background population. In Cox regression analyses corrected for age, only smoking was of significance for the progression from AP to CP. Conclusions: Acute pancreatitis can progress to CP, not only from alcoholic but also from nonalcoholic AP. Smoking was the strongest risk factor associated with progression. The mortality rate for these patients was 5 to 6 times the mortality rate in the population.