Pneumonitis has been reported to be a rare complication of low-dose MTX treatment. This paper describes a 62-year-old male patient in whom clinically successful low-dose treatment was applied. After six months of treatment, a gradually progressive pulmonary symptomatology developed and abruptly deteriorated. Radiological examination revealed interstitial and intra-alveolar densifications; functional analysis revealed partial respiratory failure. The histological finding of interstitial lymphocytic infiltration with giant cells and a chronic intra-alveolar pneumonia, with bacteriological detection of E. coli and Proteus vulg. in the BAL fluid, confirmed the suspected diagnosis of primary MTX-induced pneumonitis with secondary bacterial superinfection. In view of the fact that the literature does not seem to contain unequivocal definition, it is urgently recommended that unequivocal criteria be established.