The samples from pleural and pericardial effusion, in which immature haematopoietic cells had been identified cytologically, were re-examined. The results were then analysed along with the clinico-biological context and compared to published data. The aim was to determine the frequency, the type and the context of haematopoietic cell identification in pleural and pericardial fluid effusion. In 10 years, 28 cases were studied. Four sub-groups were described: 1) patients with a severe sepsis, 2) patients with an acute local or regional infection, 3) persistent or recurrent effusion without specific context, 4) patients who underwent a transplantation treated with cyclosporin A. Even when the clinico-pathological context did not suggest a classical extra-medullary haematopoiesis, it was not exceptional to identify immature haematopoietic cells. This hypothesis is supported by published data, which suggests that a local inflammatory state could help mesothelial cells to constitute a favourable environment for division and maturation of circulating haematopoietic progenitor cells.