The safety of azithromycin has been assessed in the clinical trial in Japan. Among the patients treated with azithromycin, side effects were recorded in 82(4.35%) of 1,886 adults and in 22(3.03%) of 726 children. The most common side effects in both groups were diarrhea, abdominal pain, and other gastrointestinal symptoms. All side effects were classed as mild or moderate. Laboratory abnormalities were recorded in 125(7.75%) of 1,279 adults and 85(19.23%) of 442 children, which included eosinophilia, neutropenia, and increases of GOT and GPT, and so on. All laboratory abnormalities were not severe and transient. Overall, azithromycin was well tolerated and can be safely used to treat bacterial infections in patients of all ages.