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Objectives: The use of fast-setting impression materials with different viscosities for the one-stage impression technique demands precise working times when mixing. We examined the effect of varying working time on impression precision in a randomized clinical trial. Materials and methods: Focusing on tooth 46, three impressions were made from each of 96 volunteers, using either a polyether (PE: Impregum Penta H/L DuoSoft Quick, 3 M ESPE) or an addition-curing silicone (AS: Aquasil Ultra LV, Dentsply/DeTrey), one with the manufacturer's recommended working time (used as a reference) and two with altered working times. All stages of the impression-taking were subject to randomization. The three-dimensional precision of the non-standard working time impressions was digitally analyzed compared to the reference impression. Statistical analysis was performed using multivariate models. Results: The mean difference in the position of the lower right first molar (vs. the reference impression) ranged from ±12 μm for PE to +19 and -14 μm for AS. Significantly higher mean values (+62 to −40 μm) were found for AS compared to PE (+21 to −26 μm) in the area of the distal adjacent tooth. Conclusions: Fast-set impression materials offer high precision when used for single tooth restorations as part of a one-stage impression technique, even when the working time (mixing plus application of the light- and heavy-body components) diverges significantly from the manufacturer's recommended protocol. Clinical relevance: Best accuracy was achieved with machine-mixed heavy-body/light-body polyether. Both materials examined met the clinical requirements regarding precision when the teeth were completely syringed with light material. [ABSTRACT FROM AUTHOR]