Nickel was measured, by electrothermal atomic absorption spectrophotometry, in sera from (a) 30 healthy adults, (b) 54 patients with acute myocardial infarction, (c) 33 patients with unstable angina pectoris without infarction, and (d) five patients with coronary atherosclerosis who developed cardiac ischemia during treadmill exercise. Mean (and SD) concentrations in Group a were 0.3 (0.3) microgram/L (range less than 0.05-1.1 microgram/L). Within 72 h after hospital admission, hypernickelemia (Ni greater than or equal to 1.2 microgram/L) was found in 41 patients of group b (76%) and in 16 patients of group c (48%). Hypernickelemia was found before and after exercise in one patient of Group d (20%). Peak values averaged 3.0 micrograms/L (range 0.4-21 micrograms/L) in Group b, 1.5 microgram/L (range less than 0.05-3.3 micrograms/L) in Group c. In Group b, the mean time interval between the peak values for creatine kinase activity and for nickel was 18 h. Serum nickel concentrations were unrelated to age, sex, time of day, cigarette smoking, medications, clinical complications, or outcome. Mechanisms and sources of release of nickel into the serum of patients with acute myocardial infarction or unstable angina pectoris are conjectural, but hypernickelemia may be related to the pathogenesis of ischemic myocardial injury.