OBJECTIVE The aim of this study is to compare vesical and renal calcium oxalate crystalluria in an attempt to correlate crystal formation with chemical composition and calcium oxalate saturation of renal urine. METHODS Urine specimens were directly collected from the bladder and the kidney, of 11 stone formers and 11 control subjects under general anesthesia. The type of crystals present in urine as well as their size, number by cubic millimeter and state of aggregation were determined. In addition, calcium, magnesium, sodium, chloride, phosphate, citrate, oxalate, pyrophosphate and uric acid were measured in order to evaluate the calcium saturation status (EQUIL V program). RESULTS Calcium oxalate crystals were detected in 3 stone formers (27%) and 2 control subjects (18%) in vesical urine and in 4 stone formers (36%) and 3 control subjects (27%) in renal urine. Only 2 stone formers presented with simultaneous renal and vesical crystalluria. Subjects of the two groups with and without renal crystalluria were compared in terms of chemical composition and calcium oxalate saturation of renal urine. Crystalluric subjects (n = 7) had significantly higher uricosuria (p = 0.02), calciuria (p = 0.04), magnesiuria (p = 0.04) and calcium oxalate molar product (p = 0.05) than noncrystalluric (n = 15); calcium oxalate saturation was similar (p = 0.5). CONCLUSIONS Beyond theorical considerations on lithogenesis, our observations and in particular the apparent discrepancy between renal and vesical crystalluria pose the problem of the clinical interest of the evaluation of calcium oxalate crystalluria based on freshly voided urine in the assessing the lithogenic risk or in the follow-up of patients who are particularly prone to stone recurrence.