Pycnodysostosis is an extremely rare genetic osteosclerosis caused by cathepsin K deficiency. We hypothesized that teriparatide, a potent anabolic agent used in the treatment of osteoporosis, might reduce skeletal fragility by activating bone turnover. We studied a typical case of pycnodysostosis in a 37-yr-old woman who exhibited short stature, skull and thorax deformities, and a history of severe fragility fractures. Cathepsin K gene sequencing was performed. Before and after 6 mo of 20 μg/d teriparatide, biochemical markers of bone turnover were measured, and 3D bone structure and microarchitecture was assessed in vivo by HR-pQCT. Qualitative and quantitative analysis of transiliac bone biopsies were performed, and the degree of mineralization was evaluated by quantitative microradiography. In vitro assessment of bone resorption was performed after separation and differentiation of CD14+ monocytes from peripheral blood. Bone structure assessed by HR-pQCT on the radius and tibia showed augmentation of cortical and trabecular density. Transiliac bone biopsy showed highly increased bone mass (+63% versus age- and sex-matched controls), a decrease in bone remodeling without evidence of active osteoblasts, and a severe decrease in the dynamic parameters of bone formation (mineralizing surfaces, −90% and bone formation rate, −93% versus age- and sex-matched controls). This depressed bone turnover probably explained the increased degree of mineralization. The presence of a novel missense mutation leading to an A141V amino acid substitution confirmed a genetic defect of cathepsin K as the cause of the disease. The deficiency of active osteoclasts was confirmed by an in vitro study that showed a decreased concentration of CD14+ monocytes (the precursor of osteoclasts) in blood. These osteoclasts had low resorptive activity when incubated on bone slices. After 6 mo of teriparatide, the structure, microarchitecture, and turnover of bone—assessed by HR-pQCT, histology, and bone turnover markers—remained unchanged. Our data strongly suggest that some features of the osteoclastic phenotype—that are absent in pycnodysostosis—are a prerequisite for the anabolic effect of PTH on osteoblasts.