Serum concentrations of beta-lactams that continuously exceed the minimum inhibitory concentration may improve therapeutic outcomes for immunosuppressed patients. The trough serum levels of ceftazidime (CAZ), cefepime (FEP) or imipenem (IMP) were prospectively determined on days 1 and 3 of treatment in cancer patients. Seventy-eight episodes of suspected infection were analysed. Trough serum levels were higher than 4 mg/L in 62%, 24% and 0% of cases in the CAZ, FEP and IMP groups, respectively, and were higher than 20 mg/L in 24% of cases in the CAZ group compared with 0% both in the FEP and IMP groups. For suspected infectious episodes in cancer patients, the traditional intermittent regimen of beta-lactams does not appear to be appropriate for the pharmacokinetic/pharmacodynamic properties of these antibiotics.