We studied the effect of oral Bacillus subtilis spore administration in a group of 95 patients with stones-associated recurrent urinary tract infections. There was a significant reduction in the frequency of infection episodes in the group treated with Bacillus subtilis spores plus antibiotics (treated group) compared with the group treated with antibiotics alone (controls). Urinary pH decreased significantly in the treated group compared with the controls. Patients with stone disease in situ had a worse prognosis. The immunomodulatory and stabilizing effect of Bacillus subtilis spores on the intestinal flora is probably responsible for this improvement.