PURPOSE Early detection of bioterrorism requires assessment of diagnoses assigned to cases of rare diseases with which clinicians have little experience. In this study, we evaluated the process of defi ning the differential diagnosis for inhalational anthrax using electronic communication within a practice-based research network (PBRN) and compared the results with those obtained from a nationwide random sample of family physicians with a mailed instrument. METHODSWe distributed survey instruments by e-mail to 55 physician members of the Wisconsin Research Network (WReN), a regional PBRN. The instruments consisted of 3 case vignettes randomly drawn from a set describing 11 patients with inhalational anthrax, 2 with infl uenza A, and 1 with Legionella pneumonia. Physicians provided their most likely nonanthrax diagnosis, along with their responses to 4 yes-or-no management questions for each case. Physicians who had not responded at 1 week received a second e-mail with the survey instrument. The comparison group consisted of the nationwide sample of physicians who completed mailed survey instruments. Primary outcome measures were response rate, median response time, and frequencies of diagnostic categories assigned to cases of inhalational anthrax. RESULTSThe PBRN response rate compared favorably with that of the national sample (47.3% vs 37.0%; P = not signifi cant). The median response time for the PBRN was signifi cantly shorter than that for the national sample (2 vs 28 days; P <.001). No signifi cant differences were found between the PBRN and the Midwest subset of the national sample in the frequencies of major diagnostic categories or in case management.CONCLUSIONS Electronic means of creating differential diagnoses for rare infectious diseases of national signifi cance is feasible within PBRNs. Information is much more rapidly acquired and is consistent with that obtained by conventional methods.