Quinolones are potent antibacterial agents that can cause drug hypersensitivity reactions affecting different organs. A better understanding of the underlying mechanism and the level of crossreactivity within different quinolones is needed to handle and prevent these diseases.The adverse side-effects caused by quinolones are the result of different immunological mechanisms and cause quite different diseases. The development of an assay detecting quinolone-specific IgE revealed specific antibodies in more than 50% of patients with immediate-type reactions, and the majority of sera also reacted with related compounds. In maculopapular drug exanthemas caused by ciprofloxacin, specific T cells could be detected and cloned. They reacted with ciprofloxacin directly, and crossreactivity to related compounds was detected in approximately 50% of the clones.Quinolones can cause drug hypersensitivity reactions by different immunological mechanisms. In-vitro analysis suggests that crossreactivity is common.