A comparative study of cultures for Mycobacterium tuberculosis was made on 24-hour and morning urine specimens. The total of 4,300 cultures included 688 cultures made from urine artificially seeded with known dilutions of M. tuberculosis and 3,612 cultures from urine routinely collected from hospitalized patients. All of the studies demonstrated a greater rate of contamination of cultures obtained from 24-hour collections of urine, and a greater recovery of both M. tuberculosis and chromogenic acid-fast bacilli from morning urine specimens. It is necessary to culture the urine frequently in cases of suspected tuberculosis of the genitourinary tract. Consequently, in view of the relative ease with which morning urine specimens can be processed and the greater percentage of recovery of M. tuberculosis from morning urine samples, it appears that there is no advantage in 24-hour collections of urine, but that morning urine specimens should be used.