This is a prospective study of wheezing children, who were followed up from the ages of seven to 21 years. Allergy factors were documented in all subjects. Hay fever and positive reactions to skin allergy tests were more prevalent in 21-year-old subjects with severe asthma. However, 50% of subjects who had ceased wheezing has hay fever, and 65% had at least one positive skin test reaction. The number of asthmatic subjects with hay fever and positive skin test reactions progressively increased between the ages of seven and 21 years. Eczema was significantly associated only with severe asthma. More 21-year-old subjects with severe asthma wee breast-fed in infancy for longer periods than in the control group and in groups with mild to moderate asthma. Further investigation is needed before the hypothesis that breast feeding protects against the development of allergic phenomena can be accepted.