The frequency in stone formers of hypomagnesuria is still controversial. It was assessed in 115 male Ca stone formers (SF) with normal renal function. Absolute hypomagnesuria (urinary Mg/creatinine (Cr) ratio <0.1) was present in less than 1% of patients. Relative (to calciuria) hypomagnesuria (RHoMg) indicated by Mg/Ca <0.5 was observed in 30.4% of patients, and using a new parameter, dMg/Cr (Nephrologie 5:101 (1984)), in only 13% of patients. As shown below, RHoMg SF selected by Mg/Ca (a) unlike those selected by dMg/Cr (b) have a higher Ca excretion than the normomagnesuric SF, indicating that Mg/Ca mainly selects hypercalciuric SF and is, therefore, not appropriate to express RHoMg (Table 1).