This is a report on 8 mediastinal tumours that occurred in young adults (19-43 years, mean: 29.4); predominantly in females (6/8). Initial symptoms consisted of thoracic pain and venectasia and in only one case in B symptoms. After surgical tumour reduction, radiation and/or chemotherapy, local recurrence was observed in each case under clinical care; abdominal spread is presently suspected in 3 patients; 3 died 11, 13 and 22 months after diagnosis. None developed leukaemia. The tumours are B-cell neoplasms with a characteristic immunophenotype: leucocyte common antigen+, common acute lymphoblastic leukaemia antigen-, B 1-antigen+, surface and cytoplasmic immunoglobulin-. Flow cytometry revealed DNA-diploidy in 7 cases and a moderately (3.2-3.8%) to extremely high (8.0-20.6%) S-phase component. The proliferation associated antigen Ki67 was detectable in 10-60% of the tumour cell nuclei, thus stressing the considerable or rapid growth. Histopathology is characterized by a diffuse growth pattern and a clearness and abundance of cytoplasm of the pleomorphic tumour cells, which vary in size and nuclear morphology from patient to patient. Apoptoses are more numerous than mitoses. Fibrosis and focal necrosis are common, sclerosis is present in 3 cases. We suggest that primary mediastinal lymphoma of B cell type is a novel B-lymphoma variant.