This paper discusses a simple method to compute sample sizes for testing interactions in analysis of variance. It uses orthogonal contrasts to determine the power. For the 2 X 2 table this gives a single degree of freedom contrast for consideration. For larger designs, one must consider a set of contrasts. The use of Bonferroni or Scheffé critical values is discussed. This method allows the user to compute easily a sample size without recourse to the non-central F distribution.