The relationship between night cough and other indices of asthma severity was studied in 21 children with clinically stable asthma and persistent night cough. Overnight cough was quantified and related to symptom scores, oxygen saturation (SaO2) during sleep, evening and morning peak flow recordings and daytime tests of lung function. In the index group the median number of coughing episodes was 23 (range 1-158). Only 4 children had counts of < 10 overnight, similar to the comparison group of 12 children all of whom had counts of < 10. There was a trend towards the association of overnight cough with reduced evening peak flow (r = -0.407, P = 0.07) and reduced SaO2 (r = -0.36, P = 0.10). Abnormalities in daytime tests of lung function were observed in 13 children. There was no relationship between night cough and daytime indices of lung function abnormality although children with more severe daytime abnormalities also had significant night cough. Conversely, five children with chronic night cough had normal daytime function. Conclusion. Night-time cough in children with asthma is not simply a reflection of daytime lung function status, whereas, overnight SaO2 correlates well. Other factors need to be explored to explain the variability of night-time cough in these children.