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The incidence rates for diseases such as cancer, cardiovascular disease, and diabetes are known to differ by ethnic/racial group. Yet, neither genetic nor environmental factors fully explain the observed differences. Failure to account for genetic expression in the absence or presence of an environmental factor and vice-versa, may lead to erroneous conclusions regarding the importance of these factors in disease etiology. In this paper, the authors present a novel method for computing sample size for case-control studies involving the interaction of genetic and environmental factors. The method is based on an indirect estimate of the odds ratio for gene-environment interaction given only the odds ratio for environmental exposure and population genotype frequency. A table is presented providing sample sizes required for detecting a minimum odds ratio for gene-environment interaction given varying genotype frequencies and environmental exposure odds ratio values. Sample size is shown to increase proportionately with genotype frequency for a given environment exposure odds ratio