We performed flexible fiberoptic bronchoscopy (FFB) on 106 heavy cigarette smokers. Six bronchial biopsy specimens, obtained from the carina and five major bronchi, were screened for squamous metaplasia. Individual biopsy specimens were sectioned into 4-microns sections, and a metaplasia index (MI), or percentage of sections containing squamous metaplasia, was determined. Metaplasia was noted at one or more biopsy sites in 66 of 99 subjects (seven were excluded from the analysis). Twenty-five percent of the subjects showed metaplasia at three or more biopsy sites, and one subject had metaplasia on all six biopsy specimens. The presence of squamous metaplasia varied from 40.4 percent in the right lower lobe to 15.3 percent in the left upper lobe. The subjects were grouped into simple categories based on the number of packs smoked per day and the pack-year history of smoking. Subjects who smoked more than two packs per day (n = 11) had the highest MI (37.4 +/- 4.9 percent, mean +/- SEM). Fifty-seven subjects smoked more than one pack per day but fewer than or equal to two packs per day, and they had a mean MI of 22.3 +/- 2.9 percent. Subjects who smoked one pack per day or less (n = 31) had a mean MI of only 12.9 +/- 2.8 percent. The MI of those who smoked more than two packs per day was significantly greater than the MI of those who smoked one pack per day or less (p < or = 0.003). While the MI varied from 12.9 +/- 3.5 percent in subjects who had smoked less than 20 pack-years to a maximum of 29.1 +/- 4.5 percent in those who had smoked greater than 60 pack-years, no statistically significant difference was detected between these two groups. Thus, we conclude that heavy tobacco use is associated with important alterations of bronchial mucosa. Furthermore, the intensity of tobacco use (packs per day) rather than the number of pack-years appears to be the more important factor in promoting squamous metaplasia of the bronchial mucosa.