A total of 386 corresponding registrations of standard ECG and 24-h Holter monitoring were compared in order to elucidate the information obtainable from a 60-s standard ECG with regard to ventricular arrhythmias. Ventricular ectopic beats (VEB) were present on 17% of the standard ECG against 90% of the monitorings (p less than 0.001). The corresponding figures for complicated VEB (multiform, repetitive, R on T) were 2% and 41%, respectively (p less than 0.001). The presence of VEB on the standard ECG was associated with a significantly increased incidence of all types of ventricular arrhythmias during Holter monitoring performed within the same day. The number of VEBs on the standard ECG did not predict the number during monitoring, but to some extent did predict the occurrence of complicated types. In 52 patients free from VEB on four consecutive standard ECGs, Holter monitoring showed complicated ectopic activity in 32 (62%). Thus standard ECG is an insensitive method for the detection of ventricular arrhythmias.