The duration of inpatient episodes due to COPD and the factors that affect it have recently been an object of increasing attention, as the aim has been to shorten inpatient periods and thereby to cut health-care costs. All hospital episodes of patients aged over 45 for a primary diagnosis of COPD equal or less than 150 days in duration were drawn from the treatment register maintained by the National Research and Development Centre for Welfare and Health. The lengths of these 152569 inpatient periods were analysed for sex, age and secondary diagnoses by covariance analysis. The mean age of men at the beginning of the hospital episode was 70.6 years and that of women 70.1 years. Men accounted for 76.9% of all inpatient episodes. Covariance analysis ofthe data with age standardised as 70.5 years yielded a mean hospital episode length of 8.9 (95% confidence interval (CI) 8.8-9.0) days. The mean length of hospital episodes without a secondary diagnosis was 7.7 (95% CI 7.6-7.7) days and that with a secondary diagnosis was 10.5 (95% CI 10.5-10.6) days. The longest inpatient episodes were recorded for the patients with secondary diagnoses of pneumonia, 14.7 (95% CI 14.2-15.2) days, and cerebral ischaemia, 14.2 (95% CI 13.5-14.9) days. Concurrent diseases prolonged the hospital episodes of COPD patients. At the beginning of a hospital episode, it is possible to estimate its duration and the need for different treatments based on the patients age and secondary diagnoses.