In a retrospective clinical trial 6056 pediatric medical records were analysed concerning the incidence of ampicillin rashes. Out of this population 1482 children of different agegroups were treated with ampicillin for various diagnoses. 44 rashes had developed in patients, equivalent to an overall incidence of rashes attributable to ampicillin of 2.9%. 907 newborns, in the age of 0 until 30 days, did not show any exanthema; elder babies, aged 31 until to 365 days, showed an incidence of rashes associated with ampicillin therapy in 4.06%. Children of the age-groups 1 to 4, and 5 to 15 years had developed skin reactions in 9.9% and 8.8%, respectively. If the large group of newborn babies is not taken in account, the average incidence of ampicillin rashes increases from 2.9% to 7.6%. Eruption of exanthema after start of ampicillin therapy occured changed between the 1st and 21st day, usually after 4 and 5 or 7 to 12 days. The mostly morbilliform rashes vanished in the average after 2 to 4 days. In the beginning of or during exanthema the number of eosinophile leucocytes was increased in 30% of patients. In 93% the ampicillin therapy was ended after skin reactions were noticed; the rashes of children, who were treated further with ampicillin, did not show any difference to those exanthema of children, whose therapy was stopped.