To determine the normal temperature of healthy infants, we studied 691 infants less than 3 months of age being seen for regularly scheduled well-baby visits. All temperatures were taken rectally with an electronic thermometer. The mean temperature was 37.5 degrees C +/- 0.3 degrees C. Using a cutoff of 2 standard deviations (SD) above the mean, fever would be defined as a temperature > or = 38.1 degrees C. The 95th percentile was 38.0 degrees C, and > or = 38.1 degrees C would correspond to values above the 95th percentile. The most widely used definition of fever at present is a temperature > or = 38.0 degrees C; by this definition, 6.5% of these well infants would be considered to have a fever. A significant rise in temperature with age was noted. For infants from birth to 30 days old, 2 SD above the mean was 38.0 degrees C; for those 31 to 60 days old, it was 38.1 degrees C; and for those 61 to 91 days old, it was 38.2 degrees C. Similarly, the 95th percentile was 37.9 degrees C, 38.0 degrees C, and 38.1 degrees C, respectively. Temperature also varied significantly with the season of the year, being higher in the summer (2 SD above a mean = 38.3 degrees C) than in the winter (2 SD above a mean = 38.0 degrees C). Fever should be defined as a temperature > or = 38.0 degrees C in infants less than 30 days of age, > or = 38.1 degrees C in 1-month-olds, and > or = 38.2 degrees C in 2-month-olds.