Objective To obtain a more rigorous estimate of the cost-effectiveness of No Smoking Day (NSD), an annual UK-wide campaign to encourage smokers to quit, than has been possible hitherto. Design Comparison of reported quit attempts in the month following NSD for three consecutive years with adjacent months using repeated national surveys of quit attempts. Setting England. Participants A total of 1309 adults who had smoked in the past year who responded to the surveys in the month following NSD (April 2007-2009) and a comparison group of 2672 adults who smoked in the past year who responded to the survey in the two adjacent months (March and May 2007-2009). Main outcome measures The number of additional smokers who quit permanently in response to NSD was estimated from the survey results. The incremental cost-effectiveness ratio (ICER) was calculated by combining this estimate with established estimates of life years gained and the known costs of NSD. Results The rate of quit attempts was 2.8 percentage points higher in the months following NSD (120/1309) compared with the adjacent months (170/2672; 95% CI 0.99% to 4.62%), leading to an estimated additional 0.07% of the 8.5 million smokers in England quitting permanently in response to NSD. The cost of NSD per smoker was (sic)0.088. The discounted life years gained per smoker in the modal age group 35-44 years was 0.00107, resulting in an ICER of (sic)82.24 (95% CI 49.7 to 231.6). ICER estimates for other age groups were similar. Conclusions NSD emerges as an extremely cost-effective public health intervention.