Twenty-four-hour urine samples were collected in two cycles as part of a study to compare the urine composition of stone formers to non-formers. In the first cycle, which spanned from 1994 to 1999, we obtained one 24-hour urine collection from 1046 participants. The second cycle began in 2003 when we invited additional stone formers and randomly selected controls to perform two 24-hour urine collections. In the first cycle, participants were ineligible if they were >70 years of age in HPFS or >65 years in NHS I or had a history of cancer or cardiovascular disease. In the second cycle, participants were ineligible if they were older than 75 years of age or had a history of cancer (other than non-melanoma skin cancer).

The rates of participation and completion among stone-forming and non-stone-forming participants in each cohort were reported previously. The 24-hour urine collection procedure used the system provided by Mission Pharmacal (San Antonio, TX). The demographic characteristics and dietary intake of participants who collected urine and those who did not were similar.