Researchers from Britain's University of Birmingham analyzed previous data from 10 studies on how long smokers survived after being diagnosed with lung cancer.
They say people with early lung cancer who continued to smoke had a 29 to 33 percent chance of surviving five years. But, those who kicked the habit had a 63 to 70 percent of living to see the five-year mark.
Researcher Amanda Parsons says the message of the study is "you should never give up on giving up." Until now, there was little proof quitting after the diagnosis made any difference to survival.
Lung cancer is the top cancer worldwide and the prognosis is usually poor. Only about 7 percent of patients make it to five years.
The study is published Friday in the BMJ, formerly known as the British Medical Journal.