Trends in phase contrast microscope (PCM) asbestos (1332214) fiber counting reported by the Proficiency Analytical Testing (PAT) program over a 20 year period were evaluated. The operation of the PAT Program was described and involved the rating and tracking of overall laboratory performance in asbestos fiber counting. Precision estimates included in a procedure used to approximate the precision of PCM asbestos fiber counting in a 1986 PAT Program were described. About one third of the PAT Program data was used to estimate intracounter variability. By dividing the data into time periods based on changes in PCM asbestos fiber counting and the PAT program, changes in performance over time were evaluated. There were three principal methods of PCM asbestos fiber counting used in the United States since the start of the PAT program in 1972. Some of the significant changes which occurred in fiber counting included changes in counting rules, changes in counter training, microscope standardization, graticules, microscope resolution tests, and quality control. Plots were made of the total variability estimates over time. Chrysotile (12001295) total variability and intercounter variability in the PAT program improved by about 35% since 1978. Two major factors accredited for this improvement included the introduction of fiber counting methods such as NIOSH P/CM 239 and the start of the AIHA industrial hygiene laboratory accreditation program. The introduction of NIOSH 7400 in 1984 did not produce a similar improvement. Factors affecting the trends in performance such as counting methods, proficiency testing and laboratory accreditation were considered.