The perimeters of mast cells and lymphocytes in human lungs were measured in electron micrographs by digitizer to determine the percentage of perimeter apposed to fibroblast (PPAF). Fibroblasts were apposed to the majority of mast cells. The median PPAF for mast cells in normal lung was 50.3 per cent, and in cryptogenic fibrosing alveolitis (CFA), 35.7 per cent. Although the value in CFA was lower, the difference was not statistically significant (median difference 11.8; 95 per cent confidence interval (-19.6, 25.1); P = 0.65). The PPAF range overall was 3.8-94.1 per cent. There was similar apposition of fibroblasts to lymphocytes, and no statistical differences were found when median PPAF results for mast cells and lymphocytes were compared for normal and CFA lung. The high degree of percentage apposition, accurately quantified in this study, shows that fibroblasts, mast cells, and lymphocytes are ideally arranged structurally in normal alveolar walls, to facilitate the many physiological interactions which are currently being uncovered. The present study also shows that apposition persists in pathological states, e.g., CFA lung, but because all these cells are increased in number in CFA lung, apposition is easier to identify here than in normal lung.