The average daily natural background dose was computed with a statistical approach from the dose records of personal dosemeters worn by professionally exposed workers in Switzerland. The values ranged between 1.3-3.0 mu Sv/day depending on the geographic location of the customers site. The highest values were found in the Lepontine Alps, the lowest in the Swiss Plateau. The results from measurements with control dosemeters at 31 customer sites were in agreement with the computed values. The geographic variation in the natural background and an inaccurate correction are likely to give rise to differences in dose records by up to 0.1 mSv/month. For quarterly surveillance periods up to 3% of the dose records are expected to be false positives or negatives.