Background From late May 2009, sporadic imported cases of novel influenza A (H1N1) were continuously confirmed in Shanghai, but there were few reports on its clinical presentation in China. The aim of the study was to investigate the demographic and clinical features of the laboratory-confirmed cases and the treatment with oseltamivir.Method We performed a retrospective study in the Shanghai Public Health Clinical Center (SHAPHC), reviewing the medical records of the laboratory-confirmed patients derived from June 10 to July 20, 2009. Results Atotal of 156 cases were enrolled, of whom 152 had a history of recent travel. The mean age was 22.6 years and 89 cases (57.1%) were males. The most common symptoms were fever, cough, and sore throat, with children more likely to run a temperature above 38.5℃ than adults. The mean leucocyte count was 5.4×10~9/L, the mean neutrophil count 3.2×10~9/L and the mean lymphocyte count 1.4×10~9/L. Other findings included a normal range or elevated level of C-reactive protein (CRP) and glutamic-pyruvic transaminase and a normal or decreased level of prealbumin; the levels of prealbumin and CRP were significantly lower in the children than in the adults. Fifty-two patients had abnormal chest CT results, with small unilateral or bilateral pulmonary infiltrates, axillary and mediastinal lymphadenopathy and local pleural thickening, while no cases showed symptoms of hypoxia. All the patients received oseltamivir and recovered without complications, but the duration of fever and virus shedding were significantly longer in the children than in the adults. Conclusions Travel-related circulation may be an important reason for the H1N1 epidemic in the non-epidemic areas, and the virus caused mild respiratory symptoms. The infection in children was more severe in terms of prealbumin levels, temperature, the duration of fever and virus shedding. Oseltamivir was effective for H1N1, but more effective in the adults than in the children.