Intra-invididual, interindividual, and analytical components of variation for each of 10 urine analytes were estimated from results obtained on analysis of urine specimens from 10 apparently healthy young men. In spot urines, urea and osmolality showed strong individuality, but intra-individual variances were larger than interindividual variances for sodium, urate, phosphate, and glucose. Urea and osmolality displayed strong individuality in daily urines, whereas sodium (when results were expressed in output terms), urate, potassium, and total protein (when results were expressed in concentration units) had intra-individual variances larger than interindividual variances. In monthly urines, calcium and glucose, in concentration terms, and phosphate, in output terms, exhibited strongest individuality. Analytical variance was a significant percentage of the total variance for total protein and glucose; the analytical methods we used for these analytes were probably unsatisfactory. A series of analytical goals for imprecision, based on biological variation, were derived. Although these goals may not be applicable in all clinical situations, they represent the beginning of a data base of in the literature.