Treatment for X-linked hypophosphatemia (XLH; vitamin D metabolites and phosphate salts) may result in hypercalcemia, hypercalciuria, nephrocalcinosis, and hyperparathyroidism. Cardiovascular abnormalities occur in association with these complications, but have not been reported in XLH. We hypothesized that such abnormalities may occur in XLH and evaluated cardiovascular status in 13 patients with this disease. All patients were asymptomatic and had normal cardiovascular physical examinations and Holter studies. Serum calcium and creatinine clearance were normal in all. However, all patients had mild to moderate nephrocalcinosis. Left ventricular hypertrophy was diagnosed by electrocardiogram in three and by ultrasonography in seven children. Baseline blood pressure (BP) was normal (mean +/- SD, 116 +/- 15/74 +/- 6 mm Hg). During exercise stress testing, systolic BP increased in all patients, but the maximal systolic pressure was less than that in healthy age- and sex-matched controls (156 +/- 20 vs. 175 +/- 23; P = 0.002, by t test). An abnormal increase in diastolic BP occurred at all levels of work load in XLH patients; their peak exercise diastolic BP was 91 +/- 12 vs. 72 +/- 6 mm Hg in controls (P < 0.0001, by t test). Whether these abnormal findings are primary defects in XLH or represent complications of treatment is unclear. Patients with XLH should be monitored closely for the development of hypertension and left ventricular hypertrophy. Investigation of the mechanisms involved and establishment of therapeutic guidelines are indicated.