Reduced osteoblast function is a primary defect in glucocorticoid-induced osteoporosis (GIO), and is reflected by decreased biochemical markers of bone formation, such as serum osteocalcin (OC) and procollagen type I N-terminal propeptide (PINP). This analysis compared the effects of teriparatide and alendronate on OC and PINP in patients with GIO. In a double-blind study, women (N=159) and men (N=38) with GIO were randomized to either teriparatide 20 μg/day by subcutaneous injection or to alendronate 10 mg/day orally. OC and PINP were measured in fasting-state serum samples obtained at baseline and at 1, 6, 18, and 36 months. Baseline bone formation markers were below the reference interval (low) in 33% of patients for OC and in 4% of patients for PINP. On teriparatide therapy, the median OC and PINP levels increased by 92% and 108%, respectively, and this resulted in only 12% and 1% of patients being low at 6 months. On alendronate therapy, the median OC and PINP levels decreased by 40% and 53%, respectively, and this resulted in 68% and 34% of patients being low at 6 months. We conclude that bone formation as determined by surrogate markers was increased in teriparatide-treated patients with GIO and decreased in alendronate-treated patients with GIO. © 2009.