Several studies have observed an association between obesity and asthma, but whether or not there is an association with rhinoconjunctivitis or eczema is unclear. To examine the relationship between body mass index categories (underweight, overweight and obesity), vigorous physical activity and television viewing and the risk of symptoms of asthma, rhinoconjunctivitis and eczema. As part of International Study of Asthma and Allergies in Childhood (ISAAC) Phase Three, parents or guardians of children aged 6-7 years completed written questionnaires about symptoms of asthma, rhinoconjunctivitis and eczema, and several potential risk factors, such as vigorous physical activity and television viewing, and other information such as the child's height and weight. Adolescents aged 13-14 years self-completed the questionnaires on these symptoms and potential risk factors and reported their own height and weight. For 28% of children and 24% of adolescents, the height and weight was objectively measured. A total of 76 164 children aged 6-7 years (from 29 centres and 17 countries) and 201 370 adolescents aged 13-14 years (from 73 centres and 35 countries) provided data meeting the inclusion criteria. There were associations between overweight and obesity, but not underweight, and symptoms of asthma and eczema but not rhinoconjunctivitis. Vigorous physical activity was positively associated with symptoms of asthma, rhinoconjunctivitis and eczema in adolescents, but not children. Viewing television for five or more hours/day was associated with an increased risk of symptoms of asthma, rhinoconjunctivitis and eczema in adolescents and symptoms of asthma in children. This study has confirmed the association between overweight and obesity and symptoms of asthma. It has extended these observations to include significant associations with symptoms of eczema, but not rhinoconjunctivitis. There are complex relationships between obesity, vigorous physical activity and sedentary behaviour and the symptoms of asthma, rhinoconjunctivitis and eczema in children. © 2012 Blackwell Publishing Ltd.