During 1 1/2 years, 768 patients-905 admissions, 342 with acute myocardial infarction (AMI), 563 with other diagnoses-were treated in the CCU at Serafimerlasarettet. On admission, both the AMI and the non-AMI group had significantly lower serum magnesium levels than a reference group. The incidence of serious VEBs, VT and VF on admission was significantly higher in the hypomagnesemic patients with AMI. AV blocks and SVB were more frequently observed in the hypermagnesemic patients, both in the AMI and in the non-AMI group. The incidence of AF and SVT was higher in the hypomagnesemic patients.