RT-PCR was used to diagnose dengue virus infections confirmed serologically in 26 returning travellers. RT-PCR was positive for three (75%) of four samples taken on or before day 3 of the illness, for 15 (78.9%) of 19 samples taken between days 4 and 7, and for none of three samples tested on or after day 8 (p 0.0337). When applied early, RT-PCR seems to be a useful tool for the diagnosis of dengue fever.