The root mean square error of approximation (RMSEA) is one of the most widely reported measures of misfit/fit in applications of structural equation modeling. When the RMSEA is of interest, so too should be the accompanying confidence interval. A narrow confidence interval reveals that the plausible parameter values are confined to a relatively small range at the specified level of confidence. The accuracy in parameter estimation approach to sample size planning is developed for the RMSEA so that the confidence interval for the population RMSEA will have a width whose expectation is sufficiently narrow. Analytic developments are shown to work well with a Monte Carlo simulation study. Freely available computer software is developed so that the methods discussed can be implemented. The methods are demonstrated for a repeated measures design where the way in which social relationships and initial depression influence coping strategies and later depression are examined. (Contains 9 figures, 6 tables and 6 footnotes.)