Lone cryptogenic fibrosing alveolitis (CFA) is histologically identical to fibrosing alveolitis associated with systemic sclerosis (FASSc), but it has a much worse prognosis after matching for disease severity at presentation. The aims of this study were to gain insights into possible pathogenetic mechanisms contributing to this prognostic difference, by comparing bronchoalveolar lavage (BAL) cellularity in the two diseases, and to evaluate the relationships between BAL findings and the regional and global extent of disease, quantified by thin-section computed tomography (CT) and lung function indices. Patients with CFA were distinguished by more extensive fibrosing alveolitis on CT (p < 0.02) and by higher counts of neutrophils (total per ml, p < 0.02; percentage p < 0.03) and eosinophils (total per ml, p < 0.002; percentages, p < 0.02) in BAL fluid. After adjustment for functional and morphologic measures of disease extent, eosinophil percentages and total counts were increased in CFA (p < 0.05 in all 12 multivariate models), but they were not independently related to regional or global disease severity. Neutrophil percentages and total counts were virtually identical in CFA and FASSc in disease of comparable severity, and they increased with increasingly extensive lobar disease and global disease, as judged by CT, p < 0.0005 in all analyses. Neutrophil levels were more closely linked to the extent of disease on CT than to the severity of functional impairment, on univariate and multivariate analysis. The higher BAL eosinophil levels seen in CFA, compared with those seen in FASSc, after adjustment for disease extent, indicate that an eosinophilic influx may be linked to the pathogenesis of fibrosing alveolitis. By contrast, BAL neutrophil levels increase with increasingly extensive disease on CT, but they do not differ independently between CFA and FASSc, suggesting that neutrophil degradation products are unlikely to account for the excess mortality in CFA, compared with that in FASSc.