Abstract • The range and distribution of Sister-Chromatid Exchange (SCE) scores in 479 control persons were determined. All SCE readings were performed in a single laboratory according to the same protocol and regularly checked by referee readers to assure consistency. A mean SCE per cell value of 9.9 and a 95th percentile of 13.4 were established for this study sample. The range of SCE scores across all non-exposed individuals tested was 5.0–17.5 SCE per cell. • Differences in SCE scores were associated with reader, smoking, sex, and, to a small extent, age. Individual test results showed reasonable consistency across the entire control group, but, as with most clinical measurements, care should be taken to avoid placing too much emphasis on a single test result when communicating with an individual. This report on the largest control group studied to date provides necessary normative data for further SCE investigations in occupational setings.