Children with Down's syndrome (DS) are more liable to vitamin D deficiency. Treating this deficiency with supplements is associated with the risk of intoxication due to increased intestinal absorption or decreased vitamin D metabolism. The aim of the study was to compare the effect of two exercise intensities on the modulation of vitamin D and Parathormone (PTH) levels in children with DS.

Forty-four DS male children aged from 8-12 years participated in the study. The subjects were assigned randomly into two equal groups. group I received high-intensity treadmill aerobic exercises (T-AE) and group II received moderate-intensity T-AE, three times per week for three months. the blood samples were collected from both groups before the intervention, after one month of intervention, then after three months of intervention to assess serum 25(OH)D and PTH levels.

Inclusion Criteria:

• all subjects were trainable and able to walk freely without assistance.
• Had vitamin D deficiency, the serum level of 25-hydroxyvitamin D (25(OH) D) was ranged from 10-20ng/mL.
• Did not take calcium or vitamin D3 supplements, medications for osteoporosis, antiepileptic drugs, or any medications that may affect the vitamin D metabolism in the last 5 months.
• The feeding method during the first two years of age for all children was bottle feeding.
• Mild to moderate mentally retarded, (IQ ranged from 45-70) to be able to understand and obey simple orders.
• free from any medical consequences for example cardiopulmonary disorders, auditory defects, and visual impairments.
• Had no previous history of strength training.
• Body mass index percentile ranged from the 50th percentile to less than the 75th percentile (healthy).
• The feeding method was by mouth with no special diet, with an absence of signs of malnutrition

Exclusion Criteria:

• autoimmune disease.
• Renal diseases.
• Have taken any thyroid medications.
• Obesity.
• epileptic fits.

All children were male