No large study has described the seasonal variation in asthma attacks in population-based asthmatics in whom sensitisation to allergen has been measured. 2637 young adults with asthma living in 15 countries reported the months in which they usually had attacks of asthma and had skin-prick tests performed. Differences in seasonal patterns by sensitisation status were assessed using generalised estimating equations. Most young adults with asthma reported periods of the year when their asthma attacks were more common (range: 47% in Sweden to 86% in Spain). Seasonal variation in asthma was not modified by sensitisation to house dust mite or cat allergens. Asthmatics sensitised to grass, birch and Alternaria allergens had different seasonal patterns to those not sensitised to each allergen, with some geographical variation. In southern Europe, those sensitised to grass allergens were more likely to report attacks occurred in spring or summer than in winter (OR March/April 2.60, 95% CI 1.70-3.97; OR May/June 4.43, 95% CI 2.34-8.39) and smaller later peaks were observed in northern Europe (OR May/June 1.25, 95% CI 0.60-2.64; OR July/August 1.66, 95% CI 0.89-3.10). Asthmatics reporting hay fever but who were not sensitised to grass showed no seasonal variations. Seasonal variations in asthma attacks in young adults are common and are different depending on sensitisation to outdoor, but not indoor, allergens.