To investigate the occurrence of infectious morbidities according to day care attendance during the first year of life.This was a cross-sectional analysis of data from the 12-month follow-up of a medium-sized city birth cohort from children born in 2015, in the Southern Brazil. Main exposure variables were day care attendance from 0 to 11 months of age, type of day care center (public or private), and age at entering day care. Health outcomes were classified as follows: "non-specific respiratory symptoms," "upper respiratory tract infection," "lower respiratory tract infection," "flu/cold," "diarrhea," or "no health problem," considering the two weeks prior to the interview administered at 12 months of life. Associations were assessed using Poisson regression adjusted by demographic, behavioral, and socioeconomic variables.The sample included 4018 children. Day care attendance was associated with all classifications of health outcomes mentioned above, except for flu/cold. These were stronger among children who entered day care at an age closer to the outcome time-point. An example are the results for lower respiratory tract infection and diarrhea, with adjusted prevalence ratios of 2.79 (95% CI: 1.67-4.64) and 2.04 (95% CI: 1.48-2.82), respectively, for those who entered day care after 8 months of age when compared with those who never attended day care.The present study consistently demonstrated the association between day care attendance and higher occurrence of infectious morbidities and symptoms at 12 months of life. Hence, measures to prevent infectious diseases should give special attention to children attending day care centers.