Objective: The aim of this study was to determine the usefulness of the morning calcium oxalate crystalluria in detecting stone formers particularly prone to recurrence. Methods: Over a 24-hour period of urine collection, the morning calcium oxalate crystalluria was evaluated as well as the risk of stone formation,established with Tiselius and Parks indices, for 25 recurrent stone formers (group 1) and 25 normal controls (group 2). Results: Morning crystalluria(type, size, number/ml and state of aggregate) and the Tiselius index were comparable in the two groups. Conversely, calciuria as well as the citrate/calcium ratio and the Parks index varied significantly for stone formers and normal controls. No particular correlation appeared between crystalluria and indices of Tiselius and Parks, calciuria, calcium-oxalate product or calcium/oxalate and citrate/calcium ratios. Conclusions: Morning calcium oxalate crystalluria does not enable an efficient characterization of recurrent stone formers. Its discordance with others potential indicators of the risk of stone formation poses the problem of their respective validity and evokes the prevalence of still unknown inhibiting agents in the phenomenon of crystallization.