Context Bariatric surgery, particularly Roux-en-Y gastric bypass (RYGB), is associated with increased risk of osteoporotic fractures. It is unknown whether RYGB or sleeve gastrectomy (SG) have different effects on bone health. Objective To compare changes in bone mineral density and markers of bone turnover one year after SG and RYGB. Design, Setting, Patients, and Interventions Randomized, triple-blind, single-center trial at a tertiary care center in Norway. Primary outcome was diabetes remission. Patients with severe obesity and type 2 diabetes were randomized and allocated (1:1) to SG or RYGB. Main Outcome Measures Changes in areal bone mineral density (aBMD) and bone turnover markers. Results Femoral neck, total hip, and lumbar spine aBMD, but not total body aBMD, decreased significantly more after RYGB (n=44) than after SG (n=48) [mean (95% CI) between group differences -2.8 % (-0.8 to -4.7), -3.0 % (-0.9 to -5.0), -4.2 % (-2.1 to -6.4), and -0.5 % (0.6 to -1.6), respectively]. The increase in procollagen type 1 N-terminal propeptide (P1NP) and C-telopeptide of type I collagen (CTX-1) were approximately 100% higher after RYGB than after SG, (both time x group, P<0.001). The changes in femoral neck, total hip and lumbar spine aBMDs and the changes in P1NP and CTX-1 were independently associated with the surgical procedure (all P<0.05) and not weight change. Conclusions RYGB was associated with greater reduction in aBMD and greater increase in bone turnover markers compared with SG. This finding could suggest greater skeletal fragility after RYGB.