In patients with unstable angina pectoris, transient episodes of ischaemia occur frequently and the prognostic implications of those predominantly silent episodes have been shown. The method of Holter tape recording is, however, not ideal for the care of unstable patients: (a) the results of a recording period are only available after this period is finished and subsequent evaluation has taken place; and (b) the sensitivity of monitoring may be increased by recording more than two leads. We have used a portable ECG computer, which monitors the 12-lead ECG every 20 s and records ECGs either when there is consistent ST deviation or at preset intervals (1, 3, 5 min). It stores the information on solid-state memory and displays a summarized ST-level trend on a monitoring screen. The stored information can be reviewed at any time at the bed-side and selected ECGs can be printed out on paper. We have evaluated this device in 23 patients on our CCU, with unstable angina (10), evolving myocardial infarction (4) and post PTCA (9). Forty-two consecutive trend recordings were controlled with independent visual interpretation of 2920 complete 12-lead ECGs. Deviations of the ST-level trend of less than 0.1 mV were due to artefact in over 95%. However, when the ST-level trend showed deviations of above 0.1 mV, 93% of the 12-lead ECGs showed true pathologic changes. Eighty-two per cent of the recorded episodes of transient ischaemia were silent; angina was in every case preceded by ECG changes. In four patients the on-line recorded ECG was instrumental for the decision towards acute angiography, PTCA or bypass-operation.(ABSTRACT TRUNCATED AT 250 WORDS)