BACKGROUND The frequency of nighttime asthma symptoms is an important measure of asthma severity. This study was designed to determine the prevalence of daytime and nighttime symptoms in adults and children with asthma and to evaluate the impact of nighttime symptoms on sleep and daytime activities. METHODS An online survey was conducted among adults (> 18 years) and mothers of children aged 2 to 17 years with asthma. The survey included questions on daytime and nighttime asthma symptoms and asthma controller medication. Invitations to complete the survey were sent to 6349 members of a global opinion panel who were identified as having asthma. Data collection was from April to May 2005. RESULTS A total of 1600 invited panelists responded to the survey. Overall, 61% of participants reported nighttime asthma symptoms and 74% reported daytime asthma symptoms. Asthma-related sleep difficulties occurred approximately 4 times per week in adults and approximately 3 times per week in children. A significantly greater proportion of adults than children reported bothersome symptoms in the morning on awakening. Wheezing and difficulty breathing were reported in a greater proportion of adults, whereas coughing was reported in a greater proportion of children. A greater proportion of adults than children reported feelings of tiredness and impaired activity on days after experiencing nighttime symptoms. Absenteeism and lateness were more commonly reported by mothers of children with asthma than by other adults. CONCLUSIONS The prevalence of reported asthma symptoms, particularly nighttime symptoms, and the effects of nighttime symptoms on sleep and daytime activities indicates that survey participants had poorly controlled asthma.