An increase in bone mineral content (BMC) as well as a decrease in the incidence of fracture of the proximal femur have been described in patients on thiazide diuretic treatment. A study was undertaken to assess, by means of dual X-ray absorptiometry, the effect of chronic low-dosage antihypertensive treatment with hydrochlorothiazide (HCT; < or = 25 mg daily) on lumbar vertebrae (representing the axial skeleton) and the nondominant radius (peripheral skeleton). Measurements were made on 27 women (mean age 63.8 +/- 8.2 years) and 23 men (65.9 +/- 9.5 years) and compared with a control group of 19 women (62 +/- 8.7 years) and 20 men (65.9 +/- 9.5 years) with minor surgical or dermatological lesions who had never taken HCT or other drugs with potential effects on mineral bone metabolism. In men who had been on HCT for more than 5 years, BMC of the lumbar vertebrae was higher by 10.3% and of the radius by 3.8% than among the controls (differences not significant). In women who had been on HCT for more than 5 years, the corresponding increases were 19.6% (P < 0.05) and 5.4% (not significant). Among those patients who had been on HCT for less than 5 years there were no significant differences compared with the control group. There were no significant differences between the groups relating as regards the proximal radius which, in contrast to the ultradistal radius, consists predominantly of cortical bone. -These data indicate that, while HCT has only a slight influence on BMC, it can be considered to have a desirable associated effect in the antihypertensive treatment of elderly patients.