The aim of this study was to characterize histologically and immunohistochemically the lung lesions developing in growing pigs, 10 and 21 days after experimental challenge with a field strain of porcine reproductive and respiratory syndrome virus (PRRSV). Lung lesions were scored for (1) pneumocyte hypertrophy and hyperplasia, (2) septal mononuclear infiltration, (3) intra-alveolar necrotic debris, (4) intra-alveolar inflammatory cell accumulation and (5) perivascular inflammatory cell accumulation. Immunohistochemistry was performed using antibodies specific for cytokeratin, Ki67, thyroid transcription factor (TTF)-1, the myelomonocytic marker MAC387 and PRRSV. Anti-TTF-1 identified type II pneumocytes and there was marked proliferation of these cells compared with control lung (P <0.05). Anti-cytokeratin labelled type I and II pneumocytes as well as bronchial epithelial cells; however, this labelling was not suitable for cell counting purposes. There was a correlation between lesion severity and the number of cells expressing Ki67 (P <0.05).