Background & Aims: The burden of diverticular disease on society is high and is increasing with an aging population. It is therefore important to identify risk factors for disease development or progression. Many lifestyle behaviors during adolescence affect risk for later disease. We searched for adolescent lifestyle factors that affect risk of diverticular disease later in life. Methods: We performed a retrospective analysis of data from 43,772 men (age, 18–20 y) conscripted to military service in Sweden from 1969 through 1970, with a follow-up period of 39 years. All conscripts underwent an extensive mental and physical health examination and completed questionnaires covering alcohol consumption, smoking, and use of recreational drugs; cardiovascular fitness was assessed using an ergometer cycle at the time of conscription. Outcome data were collected from national registers to identify discharge diagnoses of diverticular disease until the end of 2009. We performed Cox regression analysis to determine whether body mass index, cardiovascular fitness, smoking, use of recreational drugs, alcohol consumption, and risky use of alcohol, at time of conscription are independent risk factors for development of diverticular disease. Results: Overweight and obese men had a 2-fold increased risk of diverticular disease compared to normal-weight men (hazard ratio, 2.00; P <.001). A high level of cardiovascular fitness was associated with a reduced risk of diverticular disease requiring hospitalization (P =.009). Smoking (P =.003), but not use of recreational drugs (P =.11), was associated with an increased risk of diverticular disease requiring hospitalization. Risky use of alcohol, but not alcohol consumption per se, was associated with a 43% increase in risk of diverticular disease requiring hospitalization (P =.007). Conclusions: In a retrospective analysis of data from 43,772 men in Sweden, we associated being overweight or obese, a smoker, a high-risk user of alcohol, and/or having a low level of cardiovascular fitness in late adolescence with an increased risk of developing diverticular disease requiring hospitalization later in life. Improving lifestyle factors among adolescents might reduce the economic burden of diverticular disease decades later.