To study the effect of hypomagnesemia on control of atrial fibrillation (AF), serum magnesium levels were determined in 45 consecutive patients with symptomatic AF; 20% were hypomagnesemic (serum magnesium less than 1.5 mEq/liter). In a blinded treatment protocol, hypomagnesemic patients required twice the amount of intravenous digoxin to effect control of AF (p less than 0.05). Underlying diagnoses, blood chemistries and the use of other medications that could affect digoxin therapy were similar for the 2 groups. Diuretic therapy before inclusion into the study was not significantly associated with hypomagnesemia. Thus, hypomagnesemia is common among patients with symptomatic AF. Moreover, it appears to interfere with the effect of intravenous digoxin on AF. These results suggest that monitoring of serum magnesium and, where necessary, replacement of magnesium deficiency may be beneficial in patients with symptomatic AF for whom digoxin therapy is being contemplated.