The aim of this study was to verify the prevalence of bed-sharing and factors associated with this sleeping environment in an urban population in Southern Brazil. This was a cross-sectional nested cohort study with 233 mother-infant pairs selected at the maternity ward of the University Hospital in Porto Alegre. When the infant was 3 and 6 months old, home visits were performed to collect data on bed-sharing and associated variables. The main outcome was the place shared by the mother and infant for sleeping. Variables with p < 0.2 were included in a Poisson regression model. At 3 and 6 months, 31.2% and 28.5% of infants slept with their mothers at night. At 3 months, prevalence was higher for single mothers (PR: 1.56; CI: 1.01-2.39) and mothers sharing the home with the infant's maternal grandmother (PR: 1.70; CI: 1.09-2.65). Prevalence of bed-sharing at 3 months was high and associated with single mothers and sharing the home with the infant's maternal grandmother.