Twelve patients with asthma sensitized to Dermatophagoides pteronyssinus and Parietaria judaica were studied. Patients were challenged before (winter), during (summer), and after (autumn) Parietaria-pollen season. Early asthmatic reaction was positive for both allergens, and the provocative dose causing a 15% decrease of FEV1 was similar in all seasons. The provocative dose of methacholine causing a 15% decrease of FEV1 and specific IgEs for both allergens were also not significantly different throughout the time of study. Late asthmatic reaction (LAR) to Dermatophagoides occurred in eight of 12 patients in winter, in six of nine in summer, and in five of six patients in autumn. The severity of LAR, expressed as the maximum fall of FEV1 3 to 8 hours after challenge, was not significantly different between seasons. LAR to Parietaria occurred in only one of 12 patients in winter, in six of 11 in summer, and in one of six patients in autumn. In summer, LAR to Parietaria was significantly more severe than in winter. In both winter and autumn, but not in summer, LAR to Dermatophagoides was significantly more severe than to Parietaria. We conclude that LAR is allergen specific, and natural exposure has a priming effect on LAR.