Pulmonary siderosis is a disease which can be considered with the pneumoconioses of the so-called inert dust (either by deposit or by accumulation) and, from the earliest nosographic descriptions dating back to the thirties and forties, it has been substantially considered a benign form because of the scarce sclerogenic evolution and the preservation of discrete respiratory function. We present a case of pulmonary siderosis characterized on one hand by the importance and clarity of the cytopathological picture and on the other by the presence of changes, although modest, in lung function which confirm the most recent hypotheses in the literature on the potential evolution of pneumosiderotic diseases and their relatively less benign prognosis. From a preventive point of view the case draws attention to the need for greater control of environmental hygiene conditions in small factories. From an insurance and medico-legal point of view siderosis has recently been explicitly added to the occupational diseases which can be declared to the Italian state department dealing with occupational disease and disability payments (I.N.A.I.L.) (DPR of the 13.04.1994, section 47).