The pathogenesis of canine pyometra is still unclear, but bacterial infection of the endometrium, mediated by bacterial lectins, is suspected to induce pyometra. The aim of this study was to investigate sugar expression in the mucosae of the uterus and vagina of healthy dogs with normal oestrous cycles and in dogs with pyometra, using a panel of lectins to investigate the pathogenesis of pyometra. In dogs with pyometra, the uterine and vaginal mucosae were positive for lectins that selectively bind to glucose or mannose, especially during days 7–10 and 30–40 of dioestrus. These results suggest that temporal changes in sugar expression in the uterus and vagina present an opportunity for pathogens to infect the endometrium, causing pyometra.