The purpose of this study was to examine the practical importance of the correlation values between plasma and salivary cortisol levels using meta-analysis study. In this way, researchers focusing on cortisol levels in different individuals can use more systemic, practical and reliable evidence than using just salivary or plasma cortisol levels separately. For this purpose, correlational studies from 2000 up to now were collected and analyzed by meta-analysis. Results based on 18 effect sizes (n = 743) revealed that effect sizes of the correlations between plasma and salivary cortisol levels represented large effects. The overall combined effect size (Hedges's = 1.55) also represented a large effect. This finding indicates the practical importance of the correlation values for stress studies. The use of the correlation values for examining the cortisol levels in stress studies will be discussed in detail.