Gemcitabine is a purine analog with known activity in many solid tumors, namely lung, breast, pancreatic, genitourinary and head/neck cancers. Cardiac toxicity is a rare event and only one report previously described atrial fibrillation (AF) as a consequence of gemcitabine infusion. We report two cases of women suffering from lung cancer who were treated with gemcitabine. Both patients were admitted to hospital for paroxysmal AF occurring 12-24 h after the infusion of the drug. In the first case a sinus rhythm was spontaneously repristinated when AF occurred for the first time, while the second episode required an anti-arrhythmic drug to interrupt the dysrhythmia. In the second case, the patient had to be treated with digitalis glycoside to control the ventricular response without attaining a sinus rhythm. We could not recognize any other precipitating factor beyond the infusion of gemcitabine as a cause for the arrhythmia. Both cases were treated with gemcitabine for lung cancer and we observed the appearance of AF less than 24 h after drug administration. We assume that 2',2'-difluorodeoxyuridine, an active metabolite of gemcitabine, could be responsible for the toxic effect. We conclude that AF is an unusual, but potentially dangerous, side-effect of gemcitabine infusion. The arrhythmia should be suspected whenever patients complain of dyspnea and palpitations beginning 12-24 h after treatment. In these cases, the treatment of AF consists of anti-arrhythmic drugs in order to repristinate a sinus rhythm or control the heart rate.