We studied crystalluria in healthy subjects and in calcium oxalate stone formers using polarization microscopy and infrared spectroscopy, in freshly voided urine and after 48 h urine storage at +4 degrees C. 412 urine specimens from 39 normal subjects and from 172 stone formers were examined. In the latter, 90 voidings were obtained while on free diet, 61 on moderately calcium restricted diet, 34 in the fasting state and 136 while on thiazide therapy. In the normal subjects, all 91 voidings were obtained while on free diet. Crystalluria was more frequent in urine specimens collected on free diet in stone formers (48.9%) than in normal subjects (13.9%, p less than 0.001) and slightly decreased in patients on thiazide therapy (41.2%). Crystalluria increased following +4 degrees C urine storage in all groups. The most frequent crystalline phase was weddellite. Comparison of crystalline phases of calcium oxalate observed before and after +4 degrees C storage showed that crystallization spontaneously progressed to weddellite whatever the calcium oxalate phase initially present. On thiazide therapy we observed, in addition to lower calcium content, a decreased frequency of urate crystallization especially in +4 degrees C stored urine, in parallel with a decrease in urate concentration.