Background: Smokers from lower socio-economic groups are less likely to be successful in a quit attempt than more affluent smokers, even when they access smoking cessation services. Methods: Data were collected from smoking cessation service users from three contrasting areas of Great Britain-Glasgow, North Cumbria and Nottingham. Routine monitoring data were supplemented with CO-validated smoking status at 52-week follow-up and survey data on socio-economic circumstances and smoking-related behaviour. Analysis was restricted to the 2397 clients aged between 25 and 59. Results: At 52-week follow-up, 14.3% of the most affluent smokers remained quit compared with only 5.3% of the most disadvantaged. After adjustment for demographic factors, the most advantaged clients at the English sites and the Glasgow one-to-one programme were significantly more likely to have remained abstinent than those who were most disadvantaged [odds ratio: 2.5, confidence interval (CI): 1.4- 4.7 and 7.5 CI: 1.4-40.3, respectively). Mechanisms producing the inequalities appeared to include treatment compliance, household smokers and referral source. Conclusions: Rather than quitting smoking, disadvantaged smokers quit treatment. More should be done to encourage them to persevere through the first few weeks. Other causes of inequalities in quitting varied with the service provided.