As long as your child is older than 3 months and there is an obvious reason why he (or she) feels warm—the presence of a cold, for example—it is usually unnecessary to take his temperature. You should take your child's temperature, however, if he is younger than 3 months or is not breathing, drinking, or acting as he normally does. If you are concerned about these activities, speak with your doctor, whether or not you detect a fever.  Fever is defined as a body temperature greater than 100.4 degrees Fahrenheit or 38 degrees Celsius.

For a child younger than 3 months, a digital rectal thermometer is the best choice if the temperature is elevated.  It is safer than a glass thermometer and more accurate than other available thermometers. Digital pacifier thermometers are not very accurate and should not be used. You may want to begin with an underarm (axillary) thermometer to see if the body temperature is elevated. For a child between 3 months and 5 years, a digital rectal thermometer is the most accurate. An armpit (called axillary) temperature, though not as accurate as a digital rectal reading, is also acceptable in this age group. If you use axillary measurements, add 1° F to the reading to get a rough estimate of rectal temperature. An ear thermometer may be unreliable, especially in a child younger than 1 year of age. It may be used in children older than 1 year but is not as accurate as the rectal temperature. For a child older than 5 years, use an oral digital thermometer if the child is willing to cooperate by holding it under the tongue for the required time.

If your child is uncomfortable and older than 3 months, you may start anti-fever medicine at any temperature elevation. If your child is younger than 3 months, call your pediatrician before starting any anti-fever medicine. Acetaminophen is the only over-the-counter anti-fever medicine that should be used in a child younger than 6 months. Give it every four to six hours as needed. Ibuprofen may be given to children 6 months and older, every six to eight hours as needed. Both medicines should be given in a dose according to the instructions on the package or as directed by your health-care provider. With any anti-fever medicine, check the label to see whether it is appropriate for a child, what medications it contains, and the frequency with which it should be given. Be aware that some cold medicines contain acetaminophen, so read the label carefully to avoid accidentally giving your child double the correct dose of acetaminophen. Measure doses of all medicines carefully to avoid dosing errors. Do not give aspirin, in any form, to an infant or child; it has been linked to Reye syndrome, a potentially fatal illness.