In the didactic case discussed by Dr. Parillo in a recent issue of CHEST (February 2004),1reference was made to the better prognosis and relatively benign outcome of patients with Mobitz type I atrioventricular block, or Wenckebach block. While it is generally true that Mobitz type I block does not progress to higher degrees of block, at least in the short run, recently published data have suggested otherwise.2 In a study involving 147 patients with Mobitz type I block in a district general hospital, 90 patients eventually required pacemakers (for symptoms, 74 patients; for prophylaxis, 16 patients). When the main outcome measure considered was death, with higher degree block or symptomatic bradycardia as alternative measures, the 5-year survival rate to death was reduced in unpaced patients compared to the healthy population, and paced patients fared better than unpaced patients. The authors concluded that Mobitz type I block was not usually benign in those patients who were > 45 years of age, and that the majority of patients progressed to higher degrees of heart block, developed symptoms of bradycardia, or died prematurely if left unpaced. Age and the presence of bundle branch block and organic heart disease appeared to be additional risk factors. This ambiguity in the outcome of patients with Mobitz type I block has led to the issuance of conflicting guidelines for treatment, with some authorities recommending pacemaker implantation only in the presence of symptomatic bradycardia or proven infrahisian block, and others suggesting pacing when the block is present during much of the day or night, irrespective of symptoms.2 Mobitz type I block may therefore be considered to be benign only in young people or athletes.