Respiratory symptoms are common in infancy. Most illnesses occurring among children are dealt with by parents and do not require medical attention. Nevertheless, few studies have prospectively and on a community-basis assessed the amount of respiratory symptoms and general illness in normal infants. In this population-based birth cohort study, 228 healthy infants from Copenhagen, Denmark were followed from birth to 1 year of age during 2004-2006. Symptoms were registered using daily diaries and monthly home visits. Interviews were performed at inclusion and every second month. Risk factor analysis was carried out by multiple logistic regression analysis. On average, children had general symptoms for 3.5 months during their first year of life, nasal discharge being most frequent followed by cough. Frequency of all symptoms increased steeply after 6 months of age. Each child had on average 6.3 episodes (median: 5.1, inter-quartile range (IQR): 3.3-7.8) of acute respiratory tract illness (ARTI) (nasal discharge and > or = 1 of the following symptoms: cough, fever, wheezing, tachypnea, malaise, or lost appetite) and 5.6 episodes (median: 4.3, IQR: 2.1-7.3) of simple rhinitis per 365 days at risk. Determinants for respiratory symptoms were increasing age, winter season, household size, size of residence, day-care attendance, and having siblings aged 1-3 years attending a day nursery. In conclusion, the present study provides detailed data on the occurrence of disease symptoms during the first year of life in a general population cohort and emphasizes the impact of increasing age, seasonality, and living conditions on the occurrence of ARTI.