Aztreonam, the first monocyclic beta-lactam antibiotic with pure anti-Gram-negative activity, combined with flucloxacillin, a penicillinase resistant penicillin, was given as empirical treatment of 53 serious infections in very elderly people. Eighteen of the cases had positive blood cultures and 11 had a clinical picture of sepsis without positive blood cultures: Of 49 evaluable infections, 45 (92%) were cured. In 40% of the infections, antibiotic treatment could be narrowed after 72 hours to one antibiotic. Diarrhoea, mostly transitory, was the only side-effect. Aztreonam-flucloxacillin combination is a safe and effective empirical treatment regimen for serious infections in very elderly patients.