Neutropenia as a state of immunosuppression is probably the major problem in patients suffering from acute lymphoblastic leukaemia undergoing intensive chemotherapy. Fever is frequent in neutropenic patients and often related to infection. Clinically, the presence of infection in patients with neutropenia may be difficult to establish, because there are usually few signs of infection. The aim of this work was to study sensitive markers for early diagnosis of microbial infection in neutropenic children undergoing intensive chemotherapy as a treatment for acute lymphoblastic leukaemia. The study included three groups (A, B and C) of children with acute lymphoblastic leukaemia and neutropenia. Group A consisted of 29 children with febrile neutropenia and microbial infection, aged 1-14 years (5.8+/-2.9), 11 boys and 18 girls; Group B of 38 children with febrile neutropenia without microbial infection, aged 2-14 years (6.8+/-3.1), 14 boys and 24 girls; and Group C of 53 children with neutropenia without fever and without infection, aged 1-14 years (5.9+/-2.1), 21 boys and 32 girls. Blood samples were collected upon admission and before the start of any antimicrobial treatment. The samples were used for blood culture, serological tests, leukocyte count and analysis of levels of C-reactive protein, procalcitonin, total adenosine deaminase (ADA) activity and its isoenzymes, ADA-1 and ADA-2. According to our results the procalcitonin levels and total ADA activity discriminated best between neutropenic febrile (Groups A and B) and neutropenic afebrile episodes (Group C). In conclusion, this study suggests procalcitonin and total ADA activity as two easily measurable and cost effective markers for the assessment of immune response in febrile neutropenic patients with acute lymphoblastic leukaemia.