An acute acid load was used to evaluate potential chemical differences of urinary composition in recurrent oxalate stone formers and healthy controls. After intake of ammonium chloride, total calcium, ionized calcium and magnesium increased and citrate decreased significantly in both groups. Differences between stone formers and controls could be demonstrated from the excretion of total calcium, citrate, oxalate and uric acid only after acute acid load, whereas ionized calcium did not improve discrimination. These findings support the stone-promoting role of high acid food as well as the possibility of discriminating recurrent oxalate stone formers from controls by an acute acid-loading test.