In Sweden, acute gastrointestinal illness (AGI) incidence, severity, impact on productivity, related healthcare usage and associated costs are not ascertained. We measured these in 2013–2014 using a population-based cohort reporting weekly. We defined AGI as ⩾3 episodes of loose stools or vomiting/24 h; or loose stools or vomiting with ⩾2 other gastrointestinal symptoms. After each AGI episode, we collected information about perceived severity, healthcare use and absenteeism. We calculated incidence rates, AGI absenteeism and costs comprising direct healthcare costs and productivity loss due to work/school absenteeism. A total of 3241 participants reported 1696 AGI episodes [incidence 360/1000 person-years, 95% confidence interval (CI) 326–395; highest in the <5 years age group]. In the <5 years age group, 31% of episodes were perceived as mild, 61% as moderate and 8% as severe; 9·4% led to primary-care consultations, and 1·4% to hospital admissions. In the ⩾5 years age group, 18% of episodes were perceived as mild, 64% as moderate and 18% as severe; 6·4% led to primary-care consultations, and 1·9% to hospital admissions. AGI caused 8 891 000 days of absenteeism (95% CI 6 009 000–12 780 000). AGI cost €1 005 885 000 (95% CI 754 309 000–1 257 195 000) nationally for the year. In Sweden, a minority of cases perceive AGI as a mild illness. AGI is a burden on the healthcare system and causes productivity loss, with high costs. Countries may consider these estimates when prioritizing health interventions.