OBJECTIVE: To study the incidence of transfusion transmitted virus (TTV) infection in pregnant women and the route of TTV infection. METHODS: TTV DNA was detected by nested polymerase chain reaction (n PCR) technique in serum of 490 pregnant women and in umbilical blood of their infants in Guangzhou City. PCR products from the 8 women and their infants with positive TTV DNA were cloned and sequenced. RESULTS: The positive TTV DNA occurred in 87 mothers ( 17.8% ) and 12 babies born to them, with 13.8% of incidence of mother to infant transmission. The homology of nucleotide sequence of TTV isolates from Guangzhou of China and from Japan was 85.3% - 98.2% . CONCLUSIONS: There is a high incidence of TTV infection in pregant women. TTV can be transmitted via placenta (vertical transmission).