The short-term effects of corticosteroids (CS) administered intravenously (IV) on biochemical parameters of bone metabolism were followed in infants and children. Forty-nine patients from 2 months to 10 years of age, admitted to Pediatrics Department for bronchiolitis, viral-associated wheezing and croup, were treated with IV hydrocortisone or methylprednisolone (10 or 2 mg/Kg/day, respectively) for 3 days. Blood and fasting urine were collected on admission (day 1), 2 days later (day 3) and 12 days after the end of therapy (day 15). Fifty-one children of similar age and gender without respiratory problems or bone diseases were used as controls. On day 3, suppression of the bone formation markers osteocalcin (OC) (P < 0.001) and total alkaline phosphatase (ALP) (P < 0.05) was observed, but not of the bone resorption markers of hydroxyproline, pyridinoline and calcium excretion (UHyp/UCr, UPYD/UCr and UDPD/UCr, UCa/UCr). Significant decreases were indicated in serum phosphate (Pi) and the maximum renal tubular Pi reabsorption (TmP/GFR) compared to basal (P < 0.001). No significant changes were noticed in the circulating levels of calcium (Ca), parathyroid hormone (iPTH), 25OHD, 24,25(OH)2D, 1,25(OH)2D, the insulin-like growth factor-I (IGF-I) and its binding protein-3 (IGFBP-3). Two weeks after therapy, the increase of OC to higher than basal (P < 0.01) indicated a probable activation of the osteoblasts. Serum Pi and the TmP/GFR index values that had significantly decreased by day 3 returned to pretreatment levels by day 15. When assessing the effects of the CS in relation to age, no changes were detected in the levels of OC and total ALP in the <12-month-old children, but a fall of OC was observed in the >1-year-old group (P < 0.001). In contrast to the OC, the effects on serum and renal tubular reabsorption of phosphate were similar for both groups. In conclusion, short-term IV administered CS led to significant but reversible inhibition of bone formation markers, especially detectable in the >1-year-old children, without affecting the bone resorption ones. The adverse effects on phosphate metabolism were also significant, but temporal and irrespective of age.