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The aim of this study was to define the clinical and pathological features associated with lung cancer diagnosed in persons under the age of 45 compared with lung cancer in an older population of greater than 45 years.A case control study was undertaken. Cases were defined as patients diagnosed with lung cancer under the age of 45 years. Controls were lung cancer patients over 45 years matched only for the date of diagnosis. Up to four controls were selected for each case. A retrospective review was undertaken of the records of a single tertiary respiratory institution which served a population of 1.2 million.Forty-eight cases of lung cancer (< 45 years of age) were identified and were compared to 123 matched controls with lung cancer (> 45 years of age). Of the cases 67% were female compared with 32% female cases in the control group (P < 0.01). The rate of adenocarcinoma was significantly higher among cases (48% vs 27%, P = 0.001), while squamous cell carcinoma was more common in the controls (35% vs 17% P = 0.001). Smoking was common in both groups although less so among cases (79% vs 95%, P = 0.001)). There was no difference in survival rates between the cases (11 months) compared with controls (8 months) (P = 0.1).Young lung cancer patients were predominantly female and adenocarcinoma accounted for a disproportionate number of the histological types. Cigarette smoking appears to be the main aetiological agent but as 21% of the patients under 45 years were non-smokers, other factors (genetic/hormonal) may also contribute. This study has not demonstrated a worse outcome in younger patients.