Subpleural mononuclear cell infiltration. Significance in the differential diagnosis of pleuritis showing nonspecific histologic findings. To determine if patients who had lymphocyte-rich pleural effusion and a pleural biopsy without any specific findings could be histopathologically differentiated between those with tuberculous and nontuberculous pleuritis, we histologically re-evaluated the pleural biopsies of all patients whose pleural effusion was predominant with lymphocytes and contained no malignant cells. A total of 40 patients with a nonspecific histologic findings of pleural biopsy specimen were categorized based on their ultimate diagnosis as having tuberculous (n = 15), carcinomatous (n = 10) or nontuberculous, benign pleuritis (n = 15). The pleural biopsy specimen of patients with nontuberculous, benign pleuritis frequently showed a band-like infiltration of mononuclear cells in the subpleural adipose tissue with minimal pleural inflammatory infiltrate (10 out of 15 patients), while the same finding was infrequent in those with tuberculous pleuritis (0 out of 15, p = 0.0001) and pleuritis associated with carcinoma (three out of 10, p = 0.082). Based on these results, the presence of band-like infiltration of mononuclear cells in the subpleural adipose tissue with minimal pleural inflammatory infiltrate in pleural biopsy specimens of patients with lymphocyte-rich pleural effusion suggests that the pleuritis is nontuberculous in its nature. 