METHODS

The VlagtweddeNlaardingen Study is a follow-up study on chronic obstructive pulmonary disease (COPO)(20, 21). At the initial survey in Vlagtwedde, 2,243 subjects 15to 44 yr of age were investigated, and in Vlaardingen 2,449 subjects 15 to 54 yr of age were studied. Follow-up surveys took place at 3-yr intervals in 1970, 1973, 1916, 1979, 1982, and 1985 in Vlagtwedde, and in 1972, 1975, 1978, 1981, 1984, and 1987 in Vlaardingen. Histamine responsiveness was studied in a subsample of the participants (18). Questionnaire data were collected with a Dutch version of the British Medical Research Council (BMRC) questionnaire (22, 23). Subjects were considered symptomatic if at any of the surveys they had one or more of the following symptoms: cough or phlegm production on most days or nights for as many as 3 consecutive months each year;(increased) cough and phlegm for a period of at least 3 wk; shortness of breath when walking with other people of their own age on level ground; wheezing or whistling in their chest on most days or nights; or a history of attacks of shortness of breath with wheezing (" asthmatic attacks"). Ex-smokers were those who stopped smoking at least 1 month before the examina-