The effect of short term hydrochlorothiazide therapy on urinary calcium excretion was compared to that of low calcium and a combined low calcium and low sodium diet in 30 children with postglomerular hematuria. On basal conditions 9 children were normocalciuric, 11 had absorptive, 10 renal hypercalciuria. The effect of thiazide treatment on the haematuria was also evaluated. Thiazide revealed to be more effective in reducing calcium excretion than low calcium diet alone in all groups (p less than 0.001 in normocalciuria; p less than 0.01 in both hypercalciuric groups). Combined low calcium--low sodium diet and thiazide treatment were equally effective in reducing calcium excretion in the hypercalciuric groups. On the first 3 days of thiazide treatment a slight increase of hematuria was observed; in the following period a significant decrease in the occurrence (p less than 0.01 in both hypercalciuric groups) and degree (p less than 0.01 in absorptive; p less than 0.02 in renal hypercalciuria) of hematuria was noted. These data furnish further evidence on the relation of hypercalciuria and post-glomerular hematuria.