The treatment of Staphylococcus aureus septicaemia continues to be an important clinical challenge. Spikes of fever during antibiotic therapy complicate assessment of the response. In an attempt to analyse response to treatment in terms of resolution of fever we have retrospectively studied 17 patients with microbiologically proven S. aureus septicaemia. The data from temperature charts of such patients were converted to Cusum plots and analysed in order to estimate time to defervescence of fever and time to abolition of fever following the start of antibiotic therapy. Compared with conventional temperature charts, the Cusum data allowed defervescence of fever to be clearly seen. When the Cusum data were used, the time to defervescence of fever from the onset of antibiotic treatment was 21.7 h (range 8-96 h) and to abolition of fever 58.3 h (range 12-180 h). Antibiotic treatment was changed in 61% of patients on the basis of continued fever shown on the conventional temperature chart. In all of these patients, however, the Cusum plot showed clear evidence of response.