The assumption that a change in interstitial bone thickness reflects a converse change in resorption depth was recently found to be incorrect. Accordingly, we re-examined previously published data concerning trabecular thickness and wall thickness in 15 patients with nonosteomalacic osteopenia following intestinal bypass surgery for obesity. The average number of remodeling cycles completed since the operation was calculated according to two assumptions:​ First, that the measured activation frequency had been present since the operation;​ second, that activation frequency had increased in the first two years after operation because of secondary hyperparathyroidism. In comparison with mean wall thickness in 40 normal subjects (38.6 μm), resorption depth calculated in accordance with the first assumption was significantly increased (54.1 μm;​ p < 0.001), but calculated in accordance with the second assumption was unchanged (42.1 μm;​ NS). Reasons are given for believing that the second assumption is more likely to be correct than the first. Mean trabecular thickness and mean wall thickness were significantly correlated (r = 0.68;​ p < 0.005). We conclude:​ 1) Mean resorption depth cannot be inferred from interstitial bone thickness, but can be calculated if the number of remodeling cycles corresponding to the observed structural changes is known. 2) Even though interstitial bone thickness is reduced, trabecular thinning following intestinal bypass surgery is mainly due to decreased wall thickness, as the result of defects in the recruitment and/or function of osteoblasts. The same probably applies to cancellous osteopenia in various other gastrointestinal and hepatobiliary disorders. 3) The study of intestinal bone disease may shed light on the pathogenesis of other, more common, forms of osteoporosis.