When the Clinical Laboratory Improvement Act (CLIA) was passed in 1967, the Centers for Disease Control (CDC) became interested in evaluating screening performance in cytodiagnosis. Finding no validated performance measurement methods that could be used on a national scale, the CDC initiated a program of sequential investigations to develop information that would describe the state of the art in microscopic performance in gynecologic cytopathology. The first of these experiments developed a method, the Self-Assessment Workshop, to measure performance at the microscope by using sets of glass slides. This paper describes the method, its validation process and participant performance over a 15-year period. Study results indicated that cytotechnologists and pathologists tended to correctly identify specimens (slides) in the negative and benign reaction categories in up to 95% of responses, but on slides of dysplasia they made 12% of their calls too low. Carcinomas in situ and invasive squamous cancers were undercalled in only about 5% of responses, but endometrial adenocarcinomas and other rare malignancies were undercalled in as much as 20%. The self-assessment technique is a practical, useful tool for identifying cytology personnel with serious deficiencies in cell location/identification skills and is well accepted by cytotechnologists and pathologists. However its limitations should be kept in mind: screening results from this simulated test should not be extrapolated to routine work performance; the screening time limit of five minutes per slide may adversely affect performance; and, finally, these results may reflect state-of-the-art performance only in voluntary, not mandatory, settings.