Previously, characteristics of the quality assurance program in each department of pathology and laboratory medicine were the uniqueness of the monitors selected, the thresholds set for activity, and definitions of good performance. In 1989, members of the College of American Pathologists developed and implemented the first nationwide quality assurance program for pathology and laboratory medicine. This program, Q-Probes, has been expanded for 1990; after completion of planning under way for 1991, indicators will be in place for all steps in the total testing process. More than 1100 institutions had subscribed by the first quarter of 1990. Input forms for measurements of clinical indicators of quality are distributed to participants and, after completion, submitted to a central computer facility where keypunching and data manipulations occur. The interpretation of results is enhanced by peer comparisons of the participant data collected. In many circumstances studied, a large proportion of the data are aggregated at extreme ends of the distribution. Because data distributions are nongaussian for the indicators studied, it is recommended that peer comparisons use the 10% to 90% range and that the central tendency be described as the median. Despite large databases for some indicators that exceed by more than 10- to 100-fold those previously reported, extensive categorization may preclude percentile ranking of participants. Critiques are distributed describing practices and suggestions for improvement. Steady improvement of each participant and of nationwide practice patterns are documented by remeasurements.