Background: Despite evidence that patients living with cancer who continue to smoke after diagnosis are at a higher risk for all-cause mortality and reduced treatment efficacy, many cancer patients continue to smoke. This study aimed to examine the effectiveness of using a brief risk communication approach to help smokers with cancer quit smoking. Methods: A randomized controlled trial was conducted on 528 patients who continued to smoke and were follow-up at five out-patient clinics in Hong Kong. A total of 268 subjects were randomly assigned to the intervention group that received health warnings on smoking, and 260 subjects were randomly assigned to the control group that received usual care. All subjects had follow-up telephone calls at 1week, 1 month, 3 months, 6 months and 12 months to assess smoking status. Results: The biochemically validated quit rate at the 6-month follow-up was higher in the intervention group than in the control group (5.2% vs 3.8%; OR 1.38, 95% CI 0.60–3.16). The rate of at least 50% self-reported reduction of smoking at 6 months, was higher in the intervention group than in the control group (16.8% vs 12.3%; OR 1.43, 95% CI 0.88–2.35).The results showed that many smokers diagnosed with cancer believed that it was too late to quit smoking. Many participants claimed that overcoming cigarette cravings was extremely difficult, and therefore felt that the barriers to quitting outweighed the perceived benefits. Conclusions: It is crucial to proactively implement novel and effective smoking cessation interventions for smokers with cancer. Importantly, it will help to improve the physical well-being and health-related quality of life of smokers with cancer and protect the public, especially vulnerable groups such as women and children, from exposure to second-hand smoke. This will ultimately save more lives, protect the environment, and boost sustainable development. Clinical trial information: NCT01685723.