Since ions are known to influence the interaction between cells, we undertook an examination of the effect of various ions on bacterial adherence to uroepithelial cells. While most of the ions examined had no effect or decreased bacterial adherence, calcium ions significantly increased bacterial adherence. It was demonstrated, in vitro that as the concentration of calcium was increased to levels higher than normally found in the urine, there was a significant increase in bacterial adherence. It was also found that if the diet was supplemented with calcium there was an increase in the excretion of calcium in the urine and a corresponding increase in bacterial adherence when bacteria and uroepithelial cells were incubated in this urine. It is suggested that an excretion of excess calcium in the urine may lead to an increased bacterial adherence in vivo and an increased potential for urinary tract infections.