Chronic cough is a common condition that presents to both primary and secondary care. Assessment and management are hampered by the absence of well-validated outcome measures. The present study comprises the validation of the Leicester Cough Monitor (LCM), an automated sound-based ambulatory cough monitor. Cough frequency was measured with the LCM and compared with coughs and other sounds counted manually over 2 h of a 6-h recording by two observers in nine patients with chronic cough in order to determine the sensitivity and specificity of the LCM. Automated cough frequency was also compared with manual counts from one observer in 15 patients with chronic cough and eight healthy subjects. All subjects underwent 6-h recordings. A subgroup consisting of six control and five patients with stable chronic cough underwent repeat automated measurements > or = 3 months apart. A further 50 patients with chronic cough underwent 24-h automated cough monitoring. The LCM had a sensitivity and specificity of 91 and 99%, respectively, for detecting cough and a false-positive rate of 2.5 events x h(-1). Mean+/-SEM automated cough counts x patient x h(-1) was 48+/-9 in patients with chronic cough and 2+/-1 in the control group (mean difference 46 counts x patient x h(-1); 95% confidence interval (CI) 20-71). The automated cough counts were repeatable (intra-subject SD 11.4 coughs x patient x h(-1); intra-class correlation coefficient 0.9). The cough frequency in patients undergoing 24-h automated monitoring was 19 coughs x patient x h(-1); daytime (08:00-22:00 h) cough frequency was significantly greater than overnight cough frequency (25 versus 10 coughs x patient x h(-1); mean difference 15 coughs x patient x h(-1), 95% CI 8-22). The Leicester Cough Monitor is a valid and reliable tool that can be used to assess 24-h cough frequency in patients with cough. It should be a useful tool to assess patients with cough in clinical trials and longitudinal studies.