From May to July 2000, a cross-sectional study was conducted to estimate the prevalence of Trypanosoma equiperdum in the horse population of the central province (Tuv aimag) of Mongolia. On average, four herds were selected from each of the 29 aimag subdivisions (119 herds). From each herd, 10 horses were sampled in proportion to sex and age categories in the respective herds (1190 horses). Sera from 1122 horses were analysed for T. equiperdum antibodies using two serological assays, the complement fixation test (CFT) and the enzyme-linked immunosorbent assay (ELISA). The crude estimate of the CFT and the ELISA seroprevalence was 7.6 and 6.7%, respectively. Concordance between the CFT and ELISA results was high (96%). The highest number of CFT positive animals was detected in one herd in Möngönmorit (6/10), followed by herds in Bayandelger (5/10) and in Bayantsagaan (5/10). Poor body condition was significantly correlated with positive serological status in both CFT and ELISA. A history of abortion appeared to be a risk factor for both CFT and ELISA seropositivity. Blood samples of all horses belonging to herds with at least three (3/10) seropositive animals (CFT and/or ELISA) were analysed by light microscopy and by PCR using a Trypanosoma (Trypanozoon) brucei specific primer pair. No trypanosomes or any other haemoparasites could be detected in Giemsa stained thin blood smears. Eight out of the 130 samples (6.2%) analysed by PCR gave positive signals. Seven out of the eight PCR positive horses were also serologically positive. One PCR (and ELISA) positive stallion from Möngönmorit showed emaciation, scrotal and preputial oedema and an oedematous skin plaque. From the serological and DNA-based results it is concluded, that trypanosome infections occur in horses in the Tuv aimag of Mongolia. Since at present neither serological nor DNA-based tests allow a subspecies specific identification within the subgenus Trypanozoon, no definitive diagnosis can be given for T. equiperdum. Whether the examined herds are infected with T. equiperdum or with T. evansi, the causative agent of surra, remains an open question. However, based on the clinical findings, the negative parasitological results and the concentration of conspicuous seroprevalences in single herds, circumstantial evidence supports the existence of infections with the causative agent of dourine.