To characterize further the bone changes in osteoporotic patients treated by a combined calcium, vitamin D, and sodium fluoride therapy regimen, full-thickness transilial undecalcified bone biopsy specimens from ten postmenopausal white women treated for idiopathic osteoporosis for 18-24 months were compared with those from ten age-, sex-, and race-matched untreated control subjects using standard light microscopy and histomorphometry. Statistically significant bone changes in the treated group consisted of cortical and trabecular new bone formation juxtaposed on underlying normal lamellar bone (p less than 0.001). The new bone showed increased osteocytic cellularity (p less than 0.001), irregular arrangement of osteocytes (p less than 0.001), enlarged osteocyte lacunae (p less than 0.001), and periosteocytic hematoxylinophilic staining intensity (p less than 0.001). Increases were also noted in trabecular bone volume (p less than .025), trabecular osteoid surface (p less than 0.001), and trabecular osteoid volume (p less than 0.001). Osteoid calculations were significantly less than those in the clinical and chemical osteomalacia observed in the authors’ laboratory (p less than 0.01). Osteoclastic resorptive activity was increased (p less than .001), but no evidence of hyperparathyroidism was noted. These histologic and histomorphometric changes indicate accretion of new bone but with distinctly abnormal matrix characteristics. These are changes considered characteristic of the treatment and are pathologic markers of fluoride-induced abnormal bone formation.