Hodgkin's lymphoma (HL) is characterized by a few Hodgkin, Reed-Sternberg cells (HRS) surrounded by benign cells. We recently reported that mast cells were the predominant CD30L-positive cells in HL tumours, and that they activate HRS in vitro through CD30L-CD30 interaction. Here, we investigated the clinical importance of mast cell infiltration in the tumours of 123 patients. Tumour specimens were stained with a mast-cell-specific antibody that detects tryptase. Mast cells were detected in virtually every case and increasing numbers of mast cells correlated to nodular sclerosis histology (P = 0.008). Patients with higher mast cell infiltration had a worse relapse-free survival (P = 0.01).