A prospective study of 199 patients with unstable angina pectoris was undertaken to assess whether frequent serial sampling of serum creatine kinase (CK) was useful in predicting prognosis. Nineteen percent of the patients had transient CK elevations suggestive of a small myocardial infarct that was outside the detective ability of conventional electrocardiographic and enzymatic determinations. These patients had a 1 year mortality rate of 16 percent, which was significantly higher than that in the remaining patients (Fisher's exact test p = 0.05). Furthermore, the recurrence rate of myocardial infarction (14 percent) in the patients who had transient CK elevation was significantly greater than that (2 percent) in those who did not have CK elevation (Fisher's exact test p = 0/01). These data suggest that frequent serum CK sampling in the first 48 hours after admission for unstable angina has prognostic value and that persons with CK elevation may warrant a more aggressive approach to investigation and management.