Crystalluria is important in the evaluation of patients with urinary stone and is more frequently encountered in elderly than in younger adults. After noting that calcium oxalate monohydrate crystalluria was higher in elderly patients, we undertook a study to determine if oral treatment with naftidrofuryl oxalate, a drug frequently prescribed for elderly patients in France, was associated with crystalluria. The presence of early morning crystalluria was assessed in non-stone-forming patients hospitalized in a geriatric department. We studied 251 patients without a history of nephrolithiasis (mean age; 81.6 +/- 8.5 years) of whom 49 had been treated orally with naftidrofuryl oxalate at a mean dosage of 485 +/- 120 mg/24h. We identified and quantified the crystals in one early morning urine sample kept at room temperature. The frequency of crystalluria in elderly patients without stones who were not taking naftidrofuryl oxalate was 31.7% compared with only 6% in the general adult population. In this group, mainly calcium phosphate crystals were found. In patients who received naftidrofuryl oxalate, the frequency of crystalluria was 51% of which the major component was calcium oxalate monohydrate and not calcium phosphate. Naftidrofuryl oxalate may enhance crystal formation in elderly patients. This should be taken into account, particularly when other predisposing factors for nephrolithiasis are present, and a preventive increase in fluid intake considered.