Recent findings suggest that females may be more susceptible than males to the deleterious influence of tobacco smoking in developing chronic obstructive pulmonary disease (COPD). This paper studies the interaction of gender and smoking on development of COPD as assessed by lung function and hospital admission. A total of 13,897 subjects, born after 1920, from two population studies, 9,083 from the Copenhagen City Heart Study (CCHS) and 4,814 from the Glostrup Population Studies (GPS), were followed for 7-16 yrs. Data were linked with information on hospital admissions caused by COPD. Based on cross-sectional data, in the CCHS the estimated excess loss of forced expiratory volume in one second (FEV1) per pack-year of smoking was 7.4 mL in female smokers who inhaled and 6.3 mL in male smokers who inhaled. In the GPS, the corresponding excess loss of FEV1 was 10.5 and 8.4 mL in females and males, respectively. Two hundred and eighteen subjects in the CCHS and 23 in the GPS were hospitalized during follow-up. Risk associated with pack-years was higher in females than in males (relative risks (RRs) for 1-20, 20-40 and >40 pack-years were 7.0 (3.5-14.1), 9.8 (4.9-19.6) and 23.3 (10.7-50.9) in females, and 3.2 (1.1-9.1), 5.7 (2.2-14.3) and 8.4 (3.3-21.6) in males) but the interaction term gender x pack-years did not reach significance (p=0.08). Results were similar in the GPS. After adjusting for smoking in more detail, females in both cohorts had an increased risk of hospitalization for COPD compared to males with a RR of 1.5 (1.2-2.1) in the CCHS and 3.6 (1.4-9.0) in the GPS. This was not likely to be caused by a generally increased rate of hospital admission for females. Results were similar when including deaths from COPD as endpoint. In two independent population samples, smoking had greater impact on the lung function of females than males, and after adjusting for smoking females subsequently suffered a higher risk of being admitted to hospital for COPD. Results suggest that adverse effects of smoking on lung function may be greater in females than in males.