Azithromycin (AZM) is one of 15-membered rings macrolide antibiotics with wide spectrum of antimicrobial efficacy for Gram-positive and -negative bacteria and also atypical bacteria. So far, there had been no reports of the prospective studies evaluating efficacy and safety of AZM infusion in patients with mild or moderate community-acquired pneumonia (CAP). This study was conducted to evaluate prospectively the efficacy and safety of AZM in patients with mild or moderate CAP. AZM 500 mg was intravenously administered once daily, and the clinical efficacy were evaluated by clinical symptoms, peripheral blood laboratory findings and chest X-rays. Sixty-four patients were firstly registered, and eventually 61 and 62 patients were enrolled for the evaluation of clinical efficacy and safety of AZM, respectively. The efficacy of AZM in 61 patients evaluated was 88.5%. In addition, the efficacies of AZM in each pneumonia severity index by A-DROP system by the Japanese Respiratory Society (JRS) guideline in CAP were 85.2% in mild and 91.2% in moderate. Furthermore, the efficacy of AZM in each differentiation between suspicion of bacterial pneumonia and that of atypical pneumonia by JRS guideline in CAP were 91.7% in suspicion of atypical pneumonia, and its efficacy was high than that of bacterial pneumonia. Nineteen patients (20 cases; 15 with liver dysfunction, 4 with diarrhea, 1 with vascular pain) out of 62 patients were reported to have possible adverse effects of AZM. All of the patients with these adverse effects demonstrated mild dysfunction and continued AZM treatment, and these dysfunctions normalized soon after cessation of AZM. In conclusion, AZM is effective drug for patients with mild or moderate CAP, and we believe that it may be one of effective choice in the treatment of CAP patients who need hospitalization.