This study defines what degree of respiratory rate (RR) elevation can be attributed to fever using a double blind randomized pre- and post-acetaminophen comparison of vital signs of febrile children presenting to an outpatient clinic. Inclusion criteria were aged between 6 weeks and 24 months, fever between 38.5 and 40.1 degrees C, no serious illness such as sepsis, and no recent receipt of antipyretics or antibiotics. RRs counted over 1 min and rectal temperatures were recorded by a trained observer before, and 1 and 1.5 hours (hr) after receipt of 10-15 mg/kg/dose of either acetaminophen (A) or placebo (P). Randomization produced groups A (n = 54), and P (n = 50) with similar mean age (12.3 vs 12.8 mo.), gender distribution (57 vs 54% female), baseline temperature (39.1 vs 39.1 degrees C), baseline RR (44 vs 45), and hours of fever prior to visit (42 vs 37 hr). The most common diagnoses were otitis media (49%), viral syndrome (18%), upper respiratory infection (16%) or gastroenteritis (7%). The mean temperature decrement of group A was 0.4 degrees C at 1 hr and 0.9 degrees C at 1.5 hr compared to slight increases in fever of 0.3 degrees C at 1 hr and 1.5 hr in group P. Significant decreases in RR occurred in group A compared to group P at 1 hr (7.0 vs 1.9, p = 0.009) and 1.5 hr (10.8 vs 4.0, p < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)