In a prospective study of 34 hospitalized patients with moderate hypophosphatemia as an isolated electrolyte abnormality, the incidence of cardiac arrhythmias has been assessed in the absence of evident cardiac disease. Holter monitoring revealed that those without arrhythmias initially, group A, remained arrhythmia-free on follow-up. Group B (n = 8) had significant arrhythmias (Lown class II or above) and differed at p less than 0.02 from 28 controls. After phosphate replacement the arrhythmias in group B improved or disappeared (p less than 0.02). Thus, in the absence of known causes of cardiac arrhythmia, hypophosphatemia can be associated with significant ventricular ectopic activity.