Objective: To elucidate the clinical features and prognosis of multiple primary cancers, in order to make improvement of diagnosis and treatment. Methods: A total of 506 patients with two primary cancers admitted from 1973 to 2004 were analyzed retrospectively. Results: These cases accounted for 0.9% of all the hospitalized cases in the same period among which 126 were males, with the ratio of male to female 1:3. The median age at the onset of the first disease was 48 y (ranged from 24 to 77). The interval between the two cancers was longer in patients under 50 y and in males, but without statistical significance. The onset age of the two primary cancers was mainly centered around 40 to 60 y, while 70% of the second cancer occurred within 80 m after the first cancer but half of them occurred within five years. The interval between the two cancers played crucial role in affecting the prognosis (P