A 68-year-old male, having just returned from a two-week holiday on the Island of Ischia, developed unilateral pneumonia for which he was treated with oral amoxicillin-clavulanic acid and hospitalized within three days when the disease worsened and spread to both lungs. Despite parenteral amoxicillin-clavulanic acid (up to 2.2 g i.v. t.i.d.) the pneumonia spread rapidly over the next three days. Sputum cultures returned post mortem yielded Legionella pneumophila serogroup 1 and urine tests revealed the presence of Legionella antigen. Disk diffusion susceptibility testing on BCYE of the causative pathogen revealed zone diameters of inhibition of the clinical isolate exceeding 50 mm, indicating high susceptibility to this antibiotic combination. The therapeutic failure of amoxicillin-clavulanic acid should stimulate further reports and studies on the efficacy against legionellosis of this drug and similar beta-lactam inhibitor combinations as well as other beta-lactamase-stable beta-lactams.