The production of TNFα by peripheral blood mononuclear cells (PBMC) was determined in 18 hemodialysis (HD) patients. Blood was taken from each patient before and after an HD treatment. Both pre- and post-HD PBMC produced significantly more TNFα than controls (TNFα units/ml; mean ± SEM; controls 3.1 ± 0.7; pre-HD 9.7 ± 3.9; post-HD 19.8 ± 7.7, p < 0.05). In addition, post-HD PBMC produced significantly more TNFα than pre-HD PBMC suggesting that the HD procedure itself may activate cytokine production. This was true when PBMC were cultured in serum free medium as well as on culture with non-HD sera (human AB) and autologous sera. A positive correlation was also found between the production of TNFα and age in HD patients (r = 0.58; p < 0.01). Finally, normal PBMC cultured in post-HD sera produced significantly less TNFα than when cultured in the same sera pre-HD (p < 0.02). These findings suggest that PBMC of HD patients are chronically stimulated to produce TNFα which may contribute to some of the short-term and long-term complications of HD.