Achieving high follow-up rates after smoking cessation interventions (SCIs) is a general challenge. The aim of this study was to identify preferences among patients and therapists for improving follow-up rates and to assess smoking status at 6 months among patients lost to follow-up. From the Danish STOPbase for Tobacco and Nicotine, which collects data on SCI across health care, 20 representative patients lost to follow-up by routine procedures were identified together with 11 therapists. All participated in individual semi-structured phone interviews, which for patients also included 6-month smoking status. Deductive and inductive analyses were performed. Four themes emerged from the analyses with several subthemes, all regarding contacts. Both patients and therapists preferred to intensify the follow-up process by boosting it with additional attempts and using voice messages, e-mail and/or SMS, calling at specified times of the day and avoiding calls from unknown numbers. In addition, some patients mentioned that they were busy or were not carrying their mobile devices at the time of a call as a barrier. Some therapists mentioned that barriers could include an expectation of relapse, but also a poor mental state, the time of day and patient fear of public systems. Among the patients originally lost to follow-up, 35% (95% CI 16%–59%) experienced continuous smoking cessation for 6 months, and the overall national rate was 22% (21.6–23.3%). In conclusion, both patients and therapists preferred intensified follow-up. The 6-month smoking status for patients lost to follow-up seemed to be similar to that of the routinely followed-up patients. These findings will be examined experimentally in a larger study.