Since 1992, the Environmental Laboratory Approval Program of the New York State Department of Health has conducted semi-annual proficiency testing (PT) of laboratories analyzing airborne asbestos by transmission electron microscopy. While these PT rounds have included magnification measurements, diffraction measurements, zone-axis calculations, and identification of unknown mineral fibers, most samples have been dispersions of asbestos and other mineral fibers on MCE filters. Participating laboratories in each PT round were required to fill out a questionnaire detailing preparation and analytical methods. When performance of laboratories was correlated with their reported preparation and analytical methods, some significant trends were uncovered. Quality of grids was best for laboratories that 1) collapsed filters using a DMF solution, 2) used gravimetric reduction of collapsed filters to calibrate etching time, 3) etched filters for three to six minutes, and 4) used a combination of Jaffe-wick and condensation washer to dissolve filters. Reported structure concentrations were highest for laboratories that 1) etched filters for longer than six minutes, 2) evaporated carbon in a tilt/rotation geometry, and 3) performed analysis at magnifications between 15,000 and 20,000.