Eight patients (6 women and 2 men) with osteoporosis caused or aggravated by renal acidification defects are presented. Three of the female patients were premenopausal; the others were 9, 20 and 22 years postmenopausal, and two of them were on hormonal replacement therapy. Two patients had nephrolithiasis: one male with recurrent calcium phosphate stones and a left sided staghorn calculus, and one female with nephrocalcinosis due to medullary sponge kidney and hypercalciuria (patients No. 1 and 2, respectively, Table 1). In the remaining subjects, clinical suspicion was based on: a) Hip fracture in a 44-yr-old premenopausal female without any risk factor (No. 3, Table 2). b) Several vertebral compression fractures in a 45-yr-old male without hypogonadism or other predisposing factors (No. 7, Table 2). c) Lack of response to antiosteoporotic therapy in 3 women (patients No. 4, 6 and 8, Table 2). Serum bicarbonate levels and urine acidification capacity were studied in all patients. Three had low serum bicarbonate (two of whom showed high fractional excretion of bicarbonate), four had a distal defect, and one had a mixed form. Serum creatinine and potassium, and venous blood pH were normal in all cases, suggesting incomplete renal tubular acidosis. Bone mineral density in Z-score (means +/- s.e.m.) was - 1.75 +/- 0.08 in the lumbar spine (n = 8), and - 1.57 +/- 0.09 in the femoral neck (n = 4) [Tables 1 and 2; Figs 1 and 2]. Following one year treatment with oral sodium bicarbonate and potassium citrate, total skeletal calcium increased by 3-10% in five of the patients. Whereas the high prevalence of renal acidification defects among renal stone formers with or without hypercalciuria is well acknowledged, renal tubular acidosis is not included in the list of entities causing secondary osteoporosis. As shown in 6 patients of this series, incomplete RTA should be considered as another disease capable of causing osteoporosis or worsening involutional bone loss.