The relationship between rectal and peripheral-site temperature was investigated to achieve two objectives: 1) to evaluate a prevailing practice of intersite adjustment by use of an invariant temperature difference; and 2) to develop a statistical method for intersite temperature predictions in the individual child, especially for fever as defined by rectal measurement. Rectal, oral, axillary, left abdomen skin, and forehead skin temperatures (degrees F) were measured with an electronic thermometer in 257 children. Objective 1 was not achieved because a simple temperature difference between a peripheral site and the rectal site could not be used to predict rectal temperature reliably. For objective 2, intersite differences met three statistical criteria so that normal distribution theory could be used to derive the probabilities for occurrence of each difference. Accordingly, cumulative probability nomograms were constructed to estimate rectal-site fever from measurements at peripheral sites. This nomogram method produces a clinically reliable prediction of rectal-site fever from temperature measurement at peripheral sites, especially the oral and axillary sites. These predictions offer useful assessments of febrile illness severity when rectal temperature is not available.