The pattern of occurrence of mediastinal masses has to be taken into account when treating and approaching these lesions. In most reported series tumors of neural or germinal origin and thymomas are predominant. We reviewed the records of 277 patients operated for primary mediastinal lesions over the last 25 years (1965-1989) in our center. Surprisingly (Hodgkin's and non-Hodgkin's) lymphomas (24-25%) were found to emerge as the most common tumors in our series, whereas tumor of neural (11%) and germinal (65%) origins were relatively rare and decreasing in incidence. Thymomas (17%) and mediastinal cysts (14%) occurred as predicted in the literature. Consequently the malignancy rate was high (53%) even in asymptomatic patients (18%). Similar findings have been described recently by others. Factors responsible for this new distribution are difficult to identify. The therapeutic implications of the changes are discussed.