Cross-reactivity between quinolones is uncertain. Recently, we studied three patients who had developed suspected allergic reactions to a quinolone. For all of them we performed skin test, histamine release test, RAST, and oral provocation with the suspected quinolone and also with another quinolone of the opposite generation. Five atopic and five nonatopic subjects were used as controls. Neither skin test, histamine release test, nor RAST was useful in the diagnosis. By means of oral controlled provocation, the reactions were reproduced, and all the patients also reacted to another quinolone. We concluded that cross-reactivity between quinolones seems to be very important, and avoidance of any quinolone should be recommended to any patient who has suffered an allergic reaction to one of these drugs.