Nest microclimate can be influenced by subtle differences in nest location such as topography and aspect, and can differ among nests located relatively close to each other. More favorable microclimates at nests may influence birds' nest-site selection and reproductive outcome. Yet most studies of nest microclimate have considered cavity-nesting species, and few have investigated a passerine open-cup nester. We compared the nest microclimates of 87 Hooded Warbler (Setophaga citrina) nests built in subtly different topographical positions (ridge, slope, and valley) at Tar Hollow State Forest in southern Ohio. We then examined the relationships of nest topographical position and aspect to parental age and to 3 measures of reproductive outcome. We predicted that older, more experienced birds would nest in more favorable (warmer) sites and would have greater reproductive success at these sites. We found significant differences in nest microclimate among nest sites that differed subtly in topography and aspect. As expected, nest sites in valleys had significantly lower ambient temperatures (mean, minimum, and maximum daily temperatures) than those higher up on the ridges. However, nest aspect had few effects on ambient temperature at nest sites. When considering the nest sites used by females of different ages, we found some evidence that older (after second year) females had warmer nest sites than younger (second-year) females, supporting our prediction. However, there was no evidence that older females or warmer nests were associated with greater reproductive success, contrary to our expectations. Our findings suggested that although topography significantly influences nest microclimate, factors other than temperature differences are more important determinants of reproductive success, and that other considerations influence the older birds' selection of warmer nest sites on ridges. The relationships among nest microclimate, nest site selection, and reproductive success are complex and require additional study for open-cup nesters.