The variability in duplicate testings and in mother/child comparisons of RFLP VNTR data for paternity testing was analyzed. HinfI digested DNA was separated by electrophoresis in agarose gels and hybridized with radiolabelled probes detecting the VNTR-systems D7S22 (g3), D5S43 (MS8), D7S21 (MS31), D12S11 (MS43), and D2S44 (YNH24). The band positions on autoradiographs were measured with a ruler with 0.5 mm resolution. Initial analyses demonstrated that, when the samples which should be compared were investigated on the same gel, the absolute difference in migration distance was the parameter with the lowest variability. Comparisons of 445 duplicate investigations on the same gel of DNA from 108 individuals showed no differences exceeding 1.25 mm. This matching criterion was used for the comparisons of 1,012 differences in 215 mother-child pairs. All mother-child differences were less than 1.25 mm except for an assumed mutation in D7S21 (MS31), and this matching criterion has been chosen for the evaluation of Danish paternity cases. The allele distributions of the five VNTR systems in 530 unrelated Danes are presented.