Cytochrome P450 is a family of enzymes that catalyze reactions involved in the metabolism of drugs and other xenobiotics. These enzymes are therefore important in pharmacologic and toxicologic studies, and information on their abundances is of value in the process of scaling in vitro data to in vivo metabolic parameters. A meta-analysis was applied to data on the abundance of human hepatic cytochrome P450 enzymes in Caucasian adult livers (50 studies). Despite variations in the methods used to measure the abundance of enzymes, agreement between the studies in 26 different laboratories was generally good. Nonetheless, some heterogeneity was detected (Higgins and Thompson heterogeneity test). More importantly, large interindividual variability was observed in the collated data. Positive correlations between the expression levels of some cytochrome P450 enzymes were found in the abundance data, including the following pairs: CYP3A4/CYP3A5*1/*3 (Rs = 0.70, P <0.0001, n = 52), CYP3A4/CYP2C8 (Rs = 0.68, P <0.0001, n = 134), CYP3A4/CYP2C9 (Rs = 0.55, P <0.0001, n = 71), and CYP2C8/CYP2C9 (Rs = 0.55, P <0.0001, n = 99). These correlations can be used to demonstrate common genetic transcriptional mechanisms. Copyright © 2014 by The American Society for Pharmacology and Experimental Therapeutics.