Ibuprofen is 2-(4-isobutyl-fenyl)-propionic acid, which belongs to the same group of nonsteroidal antiinflammatory drugs (NSAIDs) as naproxen and ketoprofen. We present two instances of adverse reaction to pediatric ibuprofen, an acute urticaria and a fixed drug eruption, with tolerance to acetylsalicylic acid (ASA) and acetaminophen, in what seems to be hypersensitivity to the propionic acid group. Although these reactions are very rare and ibuprofen is still very safe, we think it is important to know about the possible side effects in order to recognize and treat them when they occur.