Reproducibility of a quantitative outcome is usually assessed by means of the intraclass correlation coefficient (ICC). When we are interested in assessing reproducibility from only one sample, we suggest that the study be planned with regards to the expected width of the 95 per cent confidence interval of the ICC. An approximation of this latter width is derived, which allows appraisal of the influence of n the number of subjects and p the number of replicates. Through simulation studies, the approximation is shown to be of good accuracy and can therefore be used reliably. Optimal designs are also discussed such as the optimal distribution between the number of subjects and the number of replicates per subject for a fixed total number of measures. Copyright � 2001 John Wiley & Sons, Ltd.