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Metrological performance of optical coordinate measuring machines under industrial conditions has been investigated by a comparison carried out in the period from August 2007 to January 2009. The comparison involved 21 optical CMMs of different companies in Europe, mainly equipped with image processing sensors. A set of 3 different items has been selected, including both artefacts for performance verification and common industrial workpieces. All items demonstrated adequate dimensional stability during the circulation. Measurements on the glass scale revealed a good ability of the participants in performing simple and well defined measurements of length, as well as in determining the associated measurement uncertainty. Distance measurements on the hole plate confirmed the results obtained on the glass scale. On the other hand, for holes diameter measurements, the presence of length-independent error sources was observed, with effects that were not properly managed by some participants, resulting in non-compatible measurements. Results on the plastic bricks revealed the influence of many different quantities, resulting in larger scatter of data if compared with the other artefacts. Deviations from the reference mechanical values are up to one order of magnitude larger than the corresponding CMM length measuring performance; however participants in most cases were not able to take into account the additional error sources. The comparison proved that the quality of dimensional measurement results on real industrial workpieces is largely independent on the CMM length measurement performance, as well as the limited ability of most participants to properly evaluate task-specific measurement uncertainty. Larger deviations may also be the result of the unfamiliarity of some participants with the circulated artefacts. Many industrial users of optical CMMs may be able to correctly make measurements suitable for their production parts, but struggle with the circulated artifacts, which are substantially different from their typical measurement tasks.