Sixteen patients with serious Gram-negative bacillary infections were treated with intravenous ceftazidime, 2 g every 8 h. The majority of patients had bacteraemia or pneumonitis or both. Ten patients were cured and six improved. Seven of ten patients infected with Pseudomonas aeruginosa were cured, and three improved. No adverse reactions occurred. Four strains of Ps. aeruginosa became resistant to ceftazidime in patients who were cured or improved clinically. Ceftazidime is effective as single drug therapy for serious Gram-negative infections, including those due to Ps. aeruginosa.