We assess the sex differences in mortality in a population-based cohort of those Barcelona residents older than 14 yr of age who received emergency room services (ERS) for either chronic obstructive pulmonary disease (COPD) or asthma, during the period from 1985 to 1989. Vital status was followed to the end of 1995. A total of 15,517 individuals, 9,918 males and 5,599 females were included in the study. Asthma was diagnosed in 16% of males and 53% of females. Overall, 50% of males and 30% of females died during the follow-up period. The mortality rates in both males and females who visited emergency rooms for COPD or asthma were significantly higher than the expected rates in the general population. These relative increases in the mortality rates were significantly higher in females than in males for both causes of death, COPD (age-adjusted female/male ratio = 2.39), and asthma (ratio = 3.95). However, survival was better in females than males among individuals in the study. The higher fatality in males than females was observed for all causes of death, all respiratory causes, and COPD (risk ratio among patients with COPD = 0.42, 0.29-0.59, and among patients with asthma = 0.11, 0.02-0.60), but not for asthma. Mortality for asthma was higher in females with a diagnosis of COPD (2.79, 1.52-5.13), but it was not different among individuals in whom asthma was diagnosed (1.02, 0.56-1.87). Greater severity of COPD in males than in females could explain a higher risk of dying for all respiratory causes and COPD in males. The increased risk of asthma death in females may be due to problems of coding the term "asthma" in death certificates. The higher rates in females than in males when comparing with the general population, may be an expression of a greater similarity in risk factors, such as smoking, in our population than in males and females of the general population.