Patients with nonsmall cell lung cancer (NSCLC) have been shown to have a higher prevalence of comorbidity associated with age and tobacco consumption. The objective of the present study was to determine the impact of comorbidity on survival after surgery of stage I NSCLC. In total, 588 consecutive patients operated on for a pathological stage I NSCLC between January 1, 1979 and December 31, 2003 were studied. Comorbidities were analysed individually. Overall comorbidity was assessed using the Charlson index of comorbidity (CCI). Survival data were collected for each patient from the date of operation, with a median duration of follow-up of 104 months. Survival analyses and Cox proportional hazards model analyses were used. The mean age of patients was 62.7 yrs, and 529 (89%) patients were male. The distribution of overall comorbidity severity was as follows. CCI grade 0: 47.1%; grade 1-2: 43.7%; grade 3-4: 8.3%; and grade > or =5: 0.8%. The 2, 3 and 5 yrs survival were 69, 62 and 50%, respectively. Multivariable analysis showed that T stage, age, a concomitant history of moderate-to-severe liver disease, a past history of cured cancer, cerebrovascular disease and CCI were independent predictors of survival (Hazard Ratio for CCI grade >2: 1.81; 95% confidence interval 1.25-2.63). In conclusion, comorbidity has a significant impact on survival after surgical resection of patients with stage I nonsmall cell lung cancer. The use of a validated index of comorbidity in prognostic analyses of resected nonsmall cell lung cancer is recommended.