We compared British and American patterns of ambulatory testing for chronic uncomplicated hypertensive patients by examining test use for 351 American patients cared for by 30 community-based internists in Massachusetts and 511 British patients cared for by 18 general practitioners in Greater London. For each of 13 tests examined, utilization was equal or higher for American patients. Significantly more ECGs, chest roentgenograms, plain roentgenograms (other than chest roentgenograms), blood cell counts, urinalyses, cervical cytological tests, barium enema examinations, and intravenous pyelograms were ordered. Differences ranged from four to 40 times higher in the United States. This investigation documents a marked difference in test use. Further study is needed to determine whether the conservative use of diagnostic services adversely affects patient outcomes or represents a more cost-effective form of care.