The effectiveness of acetaminophen in preventing post-vaccination fever was studied in a double-blind randomized manner. Healthy five-month-old infants vaccinated with diphtheria-tetanus-pertussis (DTP) or DTP-inactivated polio vaccine were randomly allocated to receive either placebo (n = 130) or 75 mg of acetaminophen (n = 133) four hours after the vaccination. Rectal temperatures of the infants were measured in the post-vaccination evening and next morning by the parents. The mean values of rectal temperatures were equal in both groups, i.e. 37.6 degrees C, both in the evening and in the morning. No significant difference was found in the occurrence of other minor adverse reactions. Antibody titres to diphtheria and tetanus toxoids and pertussis bacteria of the placebo (n = 25) and acetaminophen (n = 34) groups did not differ significantly from each other. It is concluded that acetaminophen in a single dose schedule is ineffective in decreasing post-vaccination fever and other symptoms.