Sudden severe life threatening attack (SSA) may occasionally develop in some patients who have had until that event a mild asthma without detectable baseline airway obstruction. Airway responsiveness (AR) measurement could be a useful mean to characterize or identify them. No data exist regarding the baseline non specific AR of such patients. We studied 5 patients having had a SSA during their past few weeks. Methacholine (MCH) challenges were performed only when standard pulmonary function tests were close to normal (FEV1 greater than or equal to 80% pred.); the patients were off medication at the time of MCH challenges. 3 to 8 evaluations were performed over a time span of 4 to 24 months. The level of AR ranged from moderately increased (PC20 = 24 mg/ml). Mean PC20 was 7,8 mg/ml. This level of AR did not differ significantly from that found in a population of 67 mild asthmatics who also were evaluated in the absence of baseline bronchial obstruction and were off medication (PC20 = 5,5mg/ml). In any individual asthmatic having sustained SSA, AR was strikingly stable. However in two patients we detected a marked decrease in PC20 which was followed within 2 weeks by a SSA. We conclude that: 1. Asthmatics who have had SSA do not demonstrate particularly AR, which is not related to the severity of their asthma. 2. Baseline AR is usually stable. 3. The measurement of AR in these patients is not very useful, specially for substantiating medication needs. 4. A marked increase in AR can be observed before SSA.