Selection of cases with additional affected relatives has been shown to increase the power of the case-control association design. We investigated whether this strategy can also improve the power of family-based association studies that use the transmission disequilibrium test (TDT), while accounting for the effects of residual polygenic and environmental factors on disease liability. Ascertainment of parent-offspring trios conditional on the proband having affected first-degree relatives almost always reduced the power of the TDT. For many disease models, this reduction was quite considerable. In contrast, for the same sample size, designs that analyzed more than one affected offspring per family often improved power when compared to the standard parent-offspring trio design. Together, our results suggest that (1) residual polygenic and environmental influences should be considered when estimating the power of the TDT for studies that ascertain families with multiple affected relatives; (2) if trios are selected conditional on having additional affected offspring, then it is important to genotype and include in the analysis the additional siblings; (3) the ascertainment strategy should be considered when interpreting results from TDT analyses. Our analytic approach to estimate the asymptotic power of the TDT is implemented online at http://pngu.mgh.harvard.edu/ ~purcell/gpc/.