OBJECTIVES--To investigate atopy and changes in symptoms, peak flow rate, and bronchial reactivity in people complaining of symptoms during the oilseed rape flowering season. METHODS--37 people who had given positive answers to questions about the presence of symptoms in relation to the flowering season of oilseed rape and 24 controls with no such symptoms were studied, although not all took part in all parts of the study. All had been previously identified in a cross sectional survey of a random sample of a rural population. Atopy was assessed by means of skin prick tests, total immunoglobulin E (IgE), and radioallergosorbent test (RAST) assays. Bronchial reactivity (PC20) was measured by histamine challenge. Total IgE and bronchial reactivity were measured both before and during the oilseed rape flowering season. Day to day changes were monitored by subjects who kept a record of their symptoms and peak flow morning and evening, starting before the flowering season and continuing during it and into the grass pollen season. RESULTS--Only two cases, of 23 tested, showed evidence of allergy to oilseed rape and only 10 of 23 tested, including these two, were atopic. Eye, nasal, and headache symptoms increased in the season in cases, which validated the questionnaire used in the previous cross sectional survey. 12 of 16 cases tested and seven of 15 controls showed a seasonal fall in PC20; the fall in the cases was significantly greater than in the controls. Peak flow charts showed no evidence of fall or of increased variability during the season. CONCLUSIONS--People who complained of symptoms in relation to the flowering of oilseed rape were rarely allergic to the plant and fewer than half were atopic. Nevertheless, they usually showed increased bronchial reactivity during the season, which may have been due in some cases to other allergens but in others to non-specific irritant effects of the air. Whether these are due to chemicals released by the crop, to rising summer ozone levels, or to other factors remains unclear.