Serum concentrations of tumour necrosis factor-alpha (TNF-alpha) increase during septicaemia in previously healthy individuals. To investigate whether a similar increase in TNF-alpha can be seen in severely immunocompromised patients with acute leukaemia and chemotherapy-induced leukopenia, serum TNF-alpha was analysed in leukopenic patients with bacterial infections. Pretherapy serum levels of TNF-alpha were decreased in leukaemia patients compared with healthy controls, and serum TNF-alpha levels showed a further decrease when patients developed chemotherapy-induced leukopenia. When leukopenic patients developed bacterial infections, serum concentrations of TNF-alpha increased. Serum levels of TNF-alpha decreased when clinical signs of infection resolved during antibiotic therapy, but an increase occurred later in parallel with haematopoietic reconstitution.