It is not certain whether clinical or laboratory findings help to distinguish true bacteraemia from contamination among acute leukaemic patients with one or more blood cultures positive for Staphylococcus epidermidis. We studied 31 patients treated at the Haematological Unit between 1 January, 1992 and 30 June, 1995 who were considered to have 'true bacteraemia', indicated by at least two positive blood cultures, and 20 considered to have probable 'contamination', indicated by a single positive culture. Fever at onset of positive blood culture, level of C-reactive protein (CRP) one day after the first positive blood culture and mortality did not differ between the groups. However, the median increase in CRP over 24 h from the first positive blood culture was significantly higher in true bacteraemias than among contaminants (median 35 mg/L vs 5 mg/L, P < 0.05). Patients with true bacteraemia were more likely than those with contaminants to have central catheters in situ (95 vs 75%, P < 0.05) and previous oral antibiotic prophylaxis (29 vs 5%, P < 0.05). Also clinical signs of catheter infection (30 vs 7%) were more common in true bacteraemias. In conclusion, central catheterization, antibiotic prophylaxis and clinical signs of catheter infection increase the likelihood of true bacteraemia; however, these factors have limited clinical utility in differentiation of true bacteraemia from contamination. Daily monitoring of serum CRP levels may help in the clinical decision-making.