To determine the role of the alveolar pores in cigarette smoke-induced lung disease, we examined the alveolar pores of guinea pigs exposed to cigarette smoke for 12 months, and compared these data to those obtained from sham-smoked animals, correlating the data with airspace size and lung function. We found that the smoke-exposed animals had a larger mean number of pores per alveolus (p < 0.001), and the distributions of pore size and shape were significantly shifted to indicate a larger and more irregular pore configuration (p < 0.001, 01 respectively). In the smoke exposed group, there was a significant correlation of pore number with total lung capacity (TLC) (0.68 p < 0.05), RV (0.70, p < 0.05), and FEV(0.1)/FVC(-0.77, p < 0.02). No correlations were identified between pore size or shape and the lung function tests. We conclude that cigarette smoke exposure produces an increase in the number of alveolar pores, a process which we believe represents ultramicroscopic emphysema. These alterations appear to precede any increase in airspace size, and may help to explain abnormal lung function in cigarette smokers without macroscopic emphysema or small airway disease. This is the first study to clearly document an increased number of alveolar pores, with a significant number of either/or large and irregular pores, after chronic smoke exposure, but in the absence of gross emphysema.