We have reviewed and compared our clinical, roentgenographic and histological material from 63 patients with sarcoidosis, berylliosis, and biological dust sensitivity. Among the histological features interstitial cellular infiltration proved to be the best for correlation with abnormal function, especially with arterial oxygen pressure and alveolar-arterial oxygen pressure difference at exercise, single-breath pressure and alveolar-arterial oxygen pressure differnce at exercise, single-breath diffusing capacity, forced vital capacity, and a composite index of overall functional impairment. There was also a significant correlation between pathological changes and a composite index of clinical severity. Some of the imperfections in these correlations may be related to strategic locations of granulomata that could alter ventilation-perfusion relationships, but part of the functional change remains unexplained by our histological measurements. Correlations of physiological and pathological features with radiographic parenchymal changes generally was poor except in the most severe cases. The finding of 4 cases with biological dust sensitivity or "allergic alveolitis" in this series reemphasizes that it is worth reviewing all cases of routinely labeled sarcoidosis in which the diagnosis has been based on the histological finding of granulomata. This is especially true in patients whose roentgenograms show no hilar nodes and minimal or "groudn-glass" changes in the lung fields despite significantly abnormal lung function and also in patients who have rapid remissions and exacerbations without therapy. Almost all of the features of berylliosis overlapped those of sarcoidosis and no distinction could be made histologically. The mean values were worse in the berylliosis group and the long-term results in treated patients were not nearly as good as in the sarcoidosis group.