Intensive training in a humid and warm environment can cause exertional heat stroke (ExHS) and rhabdomolysis (RBD) in military recruits. To investigate the role of vitamin D and monomeric calcitonin (CT) on the calcium metabolism in ExHS with RBD and acute renal failure (ARF), we studied 21 recruits with ExHS (mean age 21.4 years), 7 of which had ARF. Another 11 age-matched recruits with heat exhaustion (HE) and 11 healthy subjects were selected as controls. Our results showed that in 14 ExHS patients without ARF, mean serum creatinine (Cr) levels were significantly higher (151.16 vs. 106.08 μmol/l, p < 0.01), whereas serum osteocalcin (OC) levels were significantly lower (2.22 vs. 4.65 μg/l, p < 0.01) than in healthy controls. In 7 patients with ExHS and ARF, the mean serum Cr (774.38 vs. 105.20 μmol/l, p < 0.01), phosphorus (P) (2.26 vs. 1.26 mmol/l, p < 0.05), creatine phosphoki-nase (CPK) 274,143.97 vs. 85.78 IU/1, p < 0.05), intact parathyroid hormone (I-PTH) (299.81 vs. 18.66 ng/l, p < 0.05) and CT (13.58 vs. 6.63 ng/l, p < 0.01) levels on admission were significantly higher while the mean ionized calcium (iCa) levels were significantly lower than the healthy controls (0.9 vs. 1.18 mmol/l, p < 0.01). The mean serum 25-hydroxyvitamin D [25(OH)D] levels were not significantly different from healthy controls. However, mean serum 1,25-dihydroxyvitamin D [l,25(OH)<sub>2</sub>D] levels and the ratio of l,25(OH)<sub>2</sub>D to 25(OH)D were significantly lower than healthy controls throughout the whole course of ARF. None of the 7 patients with ExHS and ARF developed hypercalcemia during the diuretic phase. Their mean serum I-PTH levels decreased significantly from 299 to 18 ng/l during the recovery phase (p < 0.05). Our study seems to suggest that the abnormal calcium metabolism in this unique patient group is in part caused by persistently decreased renal production of l,25(OH)2D, although increased monomeric CT levels were associated with hypocalcemia. However, whether or not a causal relationship exists merits further investigation.