Populus cathayana Rehd., a dioecious tree species, is widely distributed in the northern, central and southwestern regions of China. In poplars, Melampsora larici-populina Kleb. is mainly responsible for rust disease, which is considered to be the world's most important disease of poplars. Yet, little is known about sex-related responses to rust disease in poplars. The purpose of this work was to determine whether sexually different responses occur in the antioxidant system of poplars after infection by rust disease. Three-month-old male and female P. cathayana individuals were inoculated with M. larici-populina in a greenhouse. After 12 days of incubation, we investigated the changes in physiology, biochemistry, enzyme activities and isozyme profiles. It was discovered that (i) leaf rust disease inhibited photosynthesis, caused oxidative stress and cellular membrane damage and changed antioxidant enzyme activities and isozyme profiles in poplar leaves; (ii) male poplars suffered from lower infection levels and less negative effects of leaf rust disease than did females; (iii) males showed higher antioxidant activities and less H(2)O(2) accumulation than did females after being infected by leaf rust. Thus, our results indicate that leaf rust disease is more severe in female poplars, and they suffer from greater negative effects than do males. This is the first report about sexually different responses of poplars in the antioxidant reactions to rust disease. It offers some useful information about the effect of leaf rust on dioecious plants, especially on dioecious woody plants.