Blood parasites can negatively affect energetic condition, arrival date, and reproductive performance of breeding birds, yet their impact on migrating birds is poorly understood. We quantified haematozoan infection and examined its relationship to migration timing, energetic condition, and refueling rate (plasma triglyceride concentration) for three species of wood-warblers (Parulidae) migrating through northwestern Ohio, USA from 11 April–3 June of 2004 and 2005. Prevalence of infection for magnolia Dendroica magnolia (36.3%; n=383), yellow-rumped D. coronata (58.3%; n=518), and yellow warblers D. petechia (12.7%; n=205) was approximately two times higher than reported in other migration studies and similar to infection rates found in breeding areas. Probability and intensity of infection were higher for later migrating yellow-rumped warblers, especially in younger (second-year) birds. Furthermore, we found a negative trend between intensity of infection and energetic condition for second-year yellow-rumped warblers. Haematozoan infection was not associated with refueling rate for any species. Our results indicate that for some migrants, especially younger birds, probability and intensity of haematozoan infection are related to later migration timing and reduced energetic condition during spring migration, and thus may be important determinants of subsequent reproductive performance in breeding areas.