Objective. To compare the bone anabolic drug teriparatide (20 μg/day) with the antiresorptive drug alendronate (10 mg/day) for treating glucocorticoid-induced osteoporosis (OP). Methods. This was a 36-month, randomized, double-blind, controlled trial in 428 subjects with OP (ages 22-89 years) who had received ≥5 mg/day of prednisone equivalent for ≥3 months preceding screening. Measures included changes in lumbar spine and hip bone mineral density (BMD), changes in bone biomarkers, fracture incidence, and safety. Results. Increases in BMD from baseline were significantly greater in the teriparatide group than in the alendronate group, and at 36 months were 11.0% versus 5.3% for lumbar spine, 5.2% versus 2.7% for total hip, and 6.3% versus 3.4% for femoral neck (P < 0.001 for all). In the teriparatide group, median percent increases from baseline in N-terminal type I procollagen propeptide (PINP) and osteocalcin (OC) levels were significant from 1 to 36 months (P < 0.01), and increases in levels of C-terminal telopeptide of type I collagen (CTX) were significant from 1 to 6 months (P < 0.01). In the alendronate group, median percent decreases in PINP, OC, and CTX were significant by 6 months and remained below baseline through 36 months (P < 0.001). Fewer subjects had vertebral fractures in the teriparatide group than in the alendronate group (3 [1.7%] of 173 versus 13 [7.7%] of 169; P = 0.007), with most occurring during the first 18 months. There was no significant difference between groups in the incidence of nonvertebral fractures (16 [7.5%] of 214 subjects taking teriparatide versus 15 [7.0%] of 214 subjects taking alendronate; P = 0.843). More subjects in the teriparatide group (21%) versus the alendronate group (7%) had elevated predose serum calcium concentrations (P < 0.001). Conclusion. Our findings indicate that subjects with glucocorticoid-induced OP treated with teriparatide for 36 months had greater increases in BMD and fewer new vertebral fractures than subjects treated with alendronate. © 2009, American College of Rheumatology.