Nocturnal cough in asthma is a common but poorly understood phenomenon. The aims of this study were to determine the relationship between recorded night cough, reported night cough and current wheeze in a population-based sample of children previously identified as wheezy, and to examine the relationship of nocturnal cough to current symptoms, markers of asthma severity and environmental exposure. Children were reassessed in the early school years by measuring current symptoms, ventilatory function, bronchial reactivity, peak flow variability, respiratory symptom diaries and home monitoring of overnight cough, transcutaneous arterial oxygen saturation, room temperature and humidity. Night studies were performed on 59 asymptomatic children and 41 children with current wheeze. Cough occurred more frequently in current wheezers compared to asymptomatic children (16 out of 41 (39%) vs 11 out of 59 (19%)), and more cough episodes were recorded (median 3.5 vs 2.0). Night cough was not associated with bronchial reactivity, peak flow variability, degree of morning dip, mean overnight arterial oxygen saturation, ventilatory function, maternal smoking or treatment of asthma. However, it was associated with lower overnight air temperature. Although night cough is more common in current wheezers, there is poor agreement between recorded and reported night cough. Objective tests of asthma severity are of little use in predicting its presence in this age group. The sleeping environment deserves further study.