OBJECTIVE: To test the ability of two different clinical practice guideline formats to influence physician ordering of electrodiagnostic tests in low back pain. DATA SOURCES/STUDY DESIGN: Randomized controlled trial of the effect of practice guidelines on self-reported physician test ordering behavior in response to a series of 12 clinical vignettes. Data came from a national random sample of 900 U.S. neurologists, physical medicine physicians, and general internists. INTERVENTION: Two different versions of a practice guideline for the use of electrodiagnostic tests (EDT) were developed by the U.S. Agency for Health Care Policy and Research Low Back Problems Panel. The two guidelines were similar in content but varied in the specificity of their recommendations. DATA COLLECTION: The proportion of clinical vignettes for which EDTs were ordered for appropriate and inappropriate clinical indications in each of three physician groups were randomly assigned to receive vignettes alone, vignettes plus the nonspecific version of the guideline, or vignettes plus the specific version of the guideline. PRINCIPAL FINDINGS: The response rate to the survey was 71 percent. The proportion of appropriate vignettes for which EDTs were ordered averaged 77 percent for the no guideline group, 71 percent for the nonspecific guideline group, and 79 percent for the specific guideline group (p = .002). The corresponding values for the number of EDTs ordered for inappropriate vignettes were 32 percent, 32 percent, and 26 percent, respectively (p = .08). Pairwise comparisons showed that physicians receiving the nonspecific guidelines ordered fewer EDTs for appropriate clinical vignettes than did physicians receiving no guidelines (p = .02). Furthermore, compared to physicians receiving nonspecific guidelines, physicians receiving specific guidelines ordered significantly more EDTs for appropriate vignettes (p = .0007) and significantly fewer EDTs for inappropriate vignettes (p = .04). CONCLUSIONS: The clarity and clinical applicability of a guideline may be important attributes that contribute to the effects of practice guidelines.