A pleural biopsy without granulomatous inflammation or tumour cells is interpreted as 'non-specific pleuritis' (NSP), a diagnosis without any specificity, often frustrating for physicians. However, varying histological features are found in NSPs with unknown significance. The aim of this study was to describe the detailed microscopic features of NSP and correlate them with the underlying aetiology. One hundred patients diagnosed with NSP after pleural biopsy were retrospectively evaluated. A benign cause of pleural effusion was attributed. Histological features evaluated were inflammation, fibrosis, vascular proliferation, haemorrhage, fibrin, oedema and mesothelial hyperplasia. A semi-quantitative scoring was applied. Bacterial-caused and autoimmune disease-associated NSPs showed a higher score followed by viral and drug-induced conditions, while pneumothorax and cardiac-induced NSPs showed a lower score (p<0.0001). The degree of fibrosis was higher in bacterial NSP, and the type of fibrosis was cellular in this group (p=0.006). Vascular proliferation differed between groups (p<0.0001), and was higher in bacterial NSP. Histological findings differ significantly between the varying aetiologies of NSP, and this may be used to suggest the cause of the effusion
