Long-term thiazide diuretic use is associated with higher bone mineral density and reduced hip fracture rates, which are attributed to increased serum calcium levels and decreased parathyroid activity that lead to decreased bone resorption. The present study shows that 1–100 M hydro-chlorothiazide (HCTZ) dose dependently inhibits bone resorption by isolated rat osteoclasts in the bone slice assay with an IC₅₀ of 20 M. At these concentrations, HCTZ did not affect osteoclast survival on bone slices and had no effect on the proliferation of UMR-106 rat osteoblasts, indicating that the compound is not cytotoxic. However, such concentrations of HCTZ are unlikely to be achieved in man where therapeutic doses are usually 12.5–100 mg/day. That the in vitro effect of HCTZ on bone resorption may be due to inhibition of osteoclast carbonic anhydrase is discussed.