Decreased expression of the transmembrane 4 superfamily member, CD9, is associated with poor prognosis in patients with breast or non-small cell lung cancer. The expression of CD9 in lymphoma was examined in this study. Fifty-one sections with diffuse lymphomas were examined. Thirty-seven had low expression and 14 high expression of CD9. At 5 years the progression-free survival rates were 83.3+/-10.8% and 32.8+/-9.2% (p=0.018), and the actual survival were 83.3+/-10.8% and 56.8+/-8.9% (p=0.256) for those with high and low CD9 expression respectively. Decreased expression of CD9 appears to be a prognostic factor for poor survival in patients with diffuse lymphomas.