Family-based study design is commonly used in genetic research. It has many ideal features, including being robust to population stratification (PS). With the advance of high-throughput technologies and ever-decreasing genotyping cost, it has become common for family studies to examine a large number of variants for their associations with disease phenotypes. The yield from the analysis of these family-based genetic data can be enhanced by adopting computationally efficient and powerful statistical methods. We propose a general framework of a family-based U-statistic, referred to as family-U, for family-based association studies. Unlike existing parametricbased methods, the proposed method makes no assumption of the underlying disease models and can be applied to various phenotypes (e.g., binary and quantitative phenotypes) and pedigree structures (e.g., nuclear families and extended pedigrees). By using only withinfamily information, it can offer robust protection against PS. In the absence of PS, it can also utilize additional information (i.e., betweenfamily information) for power improvement. Through simulations, we demonstrated that family-U attained higher power over a commonly used method, family-based association tests, under various disease scenarios. We further illustrated the new method with an application to large-scale family data from the Framingham Heart Study. By utilizing additional information (i.e., between-family information), family-U confirmed a previous association of CHRNA5 with nicotine dependence.KEYWORDS population stratification; pedigree structure; within-family information; between-family information; nicotine dependence B OTH population-based and family-based designs have been commonly used in genetic association studies. Case-control study is a typical population-based design, by which unrelated cases are recruited and compared with healthy controls. When cases and controls come from different ethnicity backgrounds, they could have distinct ancestry distribution, leading to false-positive association results due to population stratification (PS) (Knowler et al. 1988;Freedman et al. 2004). Many statistical methods have been proposed to address the issue of PS (Devlin and Roeder 1999;Pritchard and Rosenberg 1999; Pritchard et al. 2000a,b;Satten et al. 2001;Chen et al. 2003;Price et al. 2006;Bauchet et al. 2007). However, these methods can infer only the average effect of PS based on a large number of genetic variants, but not the locus-specific PS. Population stratification related to a particular locus may vary and deviate from the average effect. In the presence of locus-specific PS, these approaches may overadjust or underadjust the PS effect, leading to either low power or inflated type I error (Marchini et al. 2004;Qin and Zhu 2012). Unlike population-based studies, family-based studies offer robust protection against PS (Weinberg et al. 1998;Cardon and Palmer 2003;Weinberg 2003). In a typical family-based association study, the alleles transmitted to affected individu...