The standard approach in genetic-association studies is to meta-analyze the results of several genome-wide association studies (GWAS) of a single trait. However, this single-trait statistical analysis fails to exploit the information contained in GWAS analyses conducted on different but related traits. In this study, we develop a method, Multi-Trait Analysis of GWAS (MTAG), which jointly analyzes GWAS results for several related traits, thereby boosting statistical power to detect genetic associations for each of the traits.