The antipyretic effect of single doses of alpha-methyl-4-(2-thienylcarbonyl)-phenyl acetic acid (suprofen, Suprol) syrup, administered at dose levels of 1, 2, 3, 4, 5, 7.5, and 10 mg/kg b.w., was tested in a randomized double-blind and a subsequent open study. The test populations consisted of 100 children in the double-blind study and 40 patients in the open test (20 subjects/group). The patients' age ranged from 2 to 12 years; the lowest initial rectal temperature was 39.0 degrees C. The treatment groups were homogeneous as to demographic data. The temperature was reduced in all treatment groups. In the double-blind study the mean value dropped under the subfebrile threshold of 38.0 degrees C only in the group on 5 mg/kg and remained then constant for up to 6 h following administration. No sufficient antipyretic effect was obtained with lower doses. The results of the additional open study with doses of 7.5 and 10 mg/kg indicated good antipyretic effect. This effect was not, however, superior to that obtained with 5 mg/kg. Pulse and respiratory rates returned to normal within 1.5 h following administration, except in patients on 1 mg/kg. A total of 10 patients, homogeneously distributed in the treatment groups, experienced vomiting as an adverse reaction. Short-term hypotonia was seen in one subject on 7.5 mg/kg. The results obtained show that single doses of suprofen upward of 5 mg/kg b.w. exert satisfactory, long-lasting, antipyretic effect on children.