Background: High prevalence of bronchial hyperresponsiveness and asthma has been found in cross-country skiers. There is limited evidence that asthma and bronchial responsiveness would be common also in athletes with summer events. Objectives: The objective of this study was to investigate occurrence of and risk factors for increased bronchial responsiveness and asthma in elite athletes with summer events and to compare their results with those of control subjects. Methods: Forty-nine speed and power athletes (mean age 21.1 years, range 16 to 31), 71 long-distance runners (mean age 26.6 years, range 16 to 39), 42 swimmers (mean age 18.6 years, range 14 to 25), and 45 control subjects (mean age 26.7 years, range 21 to 37) were studied. The subjects answered questionnaires and were given a resting spirometric examination, a skin prick test, and a histamine challenge test. Results: Current asthma (current asthmatic symptoms and increased bronchial responsiveness) was observed in 14% (22 of 162) of the athletes and in 2% (1 of 45) of the control subjects (p = 0.041). Total asthma (current asthmatic symptoms and increased bronchial responsiveness or physician-diagnosed asthma) occurred in 23% (37 of 162) of the athletes and in 4% (2 of 45) of the control subjects (p = 0.0048). Atopy according to skin prick test results was found in 48% (77 of 162) of the athletes and in 36% (16 of 45) of the control subjects (not significant). Clinical pollen allergy (positive skin test reaction to pollen and symptoms of rhinoconjunctivitis) was significantly (p  = 0.037) more common in athletes than in control subjects. Atopic athletes showed significantly more often increased bronchial responsiveness, current asthma, and total asthma than nonatopic athletes (p = 0.011, p = 0.0049, and p < 0.0001, respectively), and the odds ratios of increased bronchial responsiveness and asthma increased with the number of positive skin test reactions. After adjustment for confounding factors, the odds ratio for the occurrence of current asthma was 5.49 (95% confidence interval 0.56 to 53.7) in speed and power athletes, 2.88 (0.30 to 27.7) in long-distance runners, and 10.8 (1.10 to 106.0) in swimmers compared with control subjects. The adjusted odds ratios for the occurrence of total asthma were 3.56 (0.62 to 20.5) in speed and power athletes, 6.01 (1.19 to 30.2) in long-distance runners, and 5.89 (1.00 to 34.5) in swimmers. Conclusions: Asthma is more common in highly trained athletes than in control subjects. Asthma is especially common in elite swimmers, but the risk of asthma is increased also in long-distance runners. Increased bronchial responsiveness and asthma are strongly associated with atopic disposition and its severity in elite athletes.(J Allergy Clin Immunol 1998;101:646-52.)