We report here two cases of Legionella pneumophila pneumonia that were markedly improved by parenteral ciprofloxacin administration. A 69-year-old man who had previously visited a hot spring was admitted to our hospital with severe pneumonia and a 48-year-old man with dilated cardiomyopathy as an underlying disease was also hospitalized because of heart failure and pneumonia. In both cases a urinary antigen test for L. pneumophila was negative at the incipient stage, and the initial treatment with a beta-lactam agent was ineffective. However, the high titer of L. pneumophila serogroup 6 antigen in the serum at the convalescent stage revealed that these two pneumonia cases were caused by L. pneumophila, and the following intravenous administration of ciprofloxacin was highly effective. We concluded that intravenous treatment with ciprofloxacin could be effective against L. pneumophila pneumonia, which is sometimes hard to diagnose in the acute phase.