The thiazide diuretics are known to cause calcium retention. In order to study the effect of thiazides on bone mineralization, we have measured the mineral content of bone at five sites (the distal radius, the distal ulna, the proximal radius, the proximal ulna, and the os calcis) in 1368 men with a mean age of 68 years, including 323 who were taking thiazides for hypertension. The results were adjusted for age and body-mass index. Thiazide users had significantly more bone mineral content at all five sites than did non-users. Untreated hypertensive patients and persons without hypertension had comparable bone mineral content, indicating that the higher mineral content found among thiazide users is related to the drug and not to the underlying hypertension. These findings suggest the possibility of a preventive or therapeutic role for thiazides in osteoporosis.