IMPORTANCE Insurers are increasingly deploying “narrow networks” with fewer contracted physicians both in health plans offered in new state exchanges under the Affordable Care Act and in Medicare Advantage (MA) plans, which are commercial alternatives offered to Medicare beneficiaries. Patients choosing health plans rely on the accuracy of network directories posted by insurers. The MA plans must meet network adequacy requirements,and inaccurate directories of participating physicians might prejudice those determinations.OBJECTIVE To determine the accuracy of MA plan directories of participating dermatologists,and the appointment availability of listed physicians.DESIGN, SETTING, AND PARTICIPANTS Scripted telephone calls were placed to every dermatologist listed in directories for the largest MA plans in 12 US metropolitan areas. The caller sought an appointment on behalf of his fictitious father who had severe itch for several months, asked whether the dermatologist accepted the relevant plan, and asked for the next available appointment date.MAIN OUTCOMES AND MEASURES Appointment availability and wait time.RESULTS Among 4754 total physician listings, 45.5%represented duplicates in the same plan directory. Among the remaining unique listings, 48.9% of physicians were reachable,accepted the listed plan, and offered an appointment for our fictitious patient. Many of the dermatologists listed had incorrect contact information, were deceased, retired, or had moved, were not accepting new patients, did not accept the insurance plan, or were subspecialized. The mean (range) wait time for appointments among the remaining listings was 45.5 (1-414) days. Both the accuracy of network directories and the appointment wait times varied substantially by health plan and metropolitan area. For 1 plan, our caller was unable to obtain an appointment with any listed dermatologist. CONCLUSIONS AND RELEVANCE Medicare Advantage physician directories for dermatology in many areas substantially overestimate the number of in-network physicians available to treat patients with medical skin conditions. These inaccuracies occurred in areas with long appointment wait times and where plans are terminating selected physician contracts. This suggests a lack of capacity that would be exacerbated by further network narrowing.Accurate physician directories are essential for proper oversight of network adequacy, and for patients who rely on these listings to evaluate health plan options during open enrollment.