We compared high-density lipoprotein cholesterol (HDL-C) and other blood and cardiovascular variables of subjects when they had been fasting overnight to those values when they had eaten breakfast. The subjects were 47 men (24 to 70 years) and 34 women (23 to 63 years) who visited our laboratory on two occasions, once fasting and once after breakfast, at the same time of the morning, exactly 1 week apart. Systolic (SBP) and diastolic (DBP) pressures and heart rates (HR) were recorded, and 24-ml blood samples were obtained from an antecubital vein and analyzed for hemoglobin (Hgb), hematocrit (Hct), HDL-C, total cholesterol (T Chol), triglycerides (Trig), total calcium (T Ca), ionized calcium (Ca++), and phosphorus (P). When subjects were fasting, their SBP, HR, Trig, T Ca, and Ca++ were lower than when they had eaten breakfast. SBP, DBP, Hgb, and Trig of the women were lower than those of the men; and HDL-C and phosphorus of the women were higher. Several correlations between variables were significant only when subjects were fasting. These were SBP with age, Hct with T Ca, and T Chol with Trig. Several correlations were significant only for one sex. For the women only, SBP, DBP, and HR were correlated with T Chol, and Trig and HDL-C were correlated with T Ca. For the men only, Hgb was correlated with HDL-C, Trig, T Ca, and P; and HDL-C was correlated with Trig.(ABSTRACT TRUNCATED AT 250 WORDS)