Abortion in dairy cows is the major factor affecting livestock development in Thailand and is caused by many diseases. Trypanosomosis is one of these factors and also results in an immunosuppressive effect in cattle. The objective of this study was to investigate the prevalence of trypanosomosis in dairy cows in central Thailand. From March to September 2007, 544 samples were collected from 105 farms in the five major dairy provinces of Kanchanaburi, Ratchaburi, Nakhon Pathom, Saraburi and Lop Buri. ELISA was performed to test all sera. The overall prevalence of T evansi infection in dairy cows was 8.1% (44/544) and herd prevalence was 19.2% (20/105). The highest individual prevalence was found at Saraburi (17.4%. 21/121) but the highest number of herd infections was at Nakhon Pathom (30%, 6/20). The parity-four and four-plus cows were 3.7 times more likely to be infected than heifers and parity-one cows (P<0.034). 1 arge herds (40 milking cows) were found to be 5.4 times more infected than small herds (P4.021). The results found that trypanosome infection might be the predisposing cause of other diseases and is a barrier to productivity gains in dairy herds.