The recently discovered parathyroid hypertensive factor (PHF) has been shown to increase intracellular free calcium levels. High intracellular calcium can cause insulin resistance, as seen in non-insulin-dependent diabetes mellitus (NIDDM) patients. We therefore compared plasma PHF activity in NIDDM patients and nondiabetic control subjects using a rat bioassay. More NIDDM patients (65%) than non-diabetic controls (33.3%) had detectable PHF activity. When injected into rats, plasma of 185 NIDDM patients caused an average increase of 4.89 ± 0.68 mm Hg in rat mean arterial pressure (MAP). This change was significantly greater than a slight drop of 0.42 ± 0.96 mm Hg (mean ± SE) for the 127 nondiabetic control subjects (Student's t test, p < 0.001). A higher percentage of the NIDDM patients than of the non-diabetic patients were hypertensive. However, a stratified analysis showed that diabetic patients consistently had significantly higher PHF levels than did nondiabetic patients, regardless of being hypertensive or not. PHF activity did not correlate with age, sex, body mass index, or the type of hypoglycemic agent taken. However, multivariate logistic regression analysis showed a positive correlation between cholesterol and PHF level among the diabetic patients (p = 0.03), but such a correlation did not exist among the non-diabetic controls. Our data indicate that elevated PHF may be responsible for insulin resistance in a fraction of NIDDM patients and suggest that PHF may be related to serum cholesterol levels in NIDDM.