Background: There is evidence that children experience a transient increase in mild episodes of gastroenteritis when they start attending out-of-home child care. We assessed the transient and sustained effects of cumulative day-care attendance, age at first enrolment and type of child care facility attended on hospitalization rates for gastroenteritis. Methods: Cox proportional hazard models were used to estimate the risk of being hospitalized for gastroenteritis in two large cohorts of preschool (6 years old) and elementary school-going (6-10 years old) children in Denmark. Day-by-day child-level registry data were used. Together, the two cohorts comprised 443 872 children, 21 038 hospitalizations and 1742 284 child-years (1994-2011). Results: From first day-care attendance until 12 months of cumulated attendance, preschool children attending day-care centres, but not those attending day-care homes, had an increased risk of hospitalization for gastroenteritis compared with children never attending registered day-care. Such increased risk was highest shortly after starting daycare attendance and then gradually declined. After 12 months of attendance, attending either day-care centres or day-care homes was associated with a lower risk for hospitalization. Such decreased risk was confined to children starting day-care attendance before the age of one year and extended throughout, but not beyond, their preschool years. Conclusions: Attending day-care centres is associated with a higher risk for gastroenteritis hospitalization until completing 1 year of attendance. However, if children start attending day-care before the age of 1 year, they experience a lower risk of being hospitalized during their preschool years. This apparent protective effect does not last into the elementary school years. (author abstract)