The aim of this study was to examine the reliability of eight methods for determining the mesio-distal crown diameters (MDD) of the permanent canines and premolars (C, P(1), and P(2)). The study models of 120 children (60 boys and 60 girls) aged from 14 to 18 years treated in orthodontic clinics were measured using Seipel's method, with callipers accurate to 0.01 mm. The correlation coefficients were determined between the MDD of C, P(1), and P(2) measured on the study models and the sums calculated using various other methods. The correlation coefficients derived by comparison with the methods of Moyers, Tanka and Johnston, Droschl et al., Legović and Hauz, and Berendonk-Nawrath showed a tendency to over-estimate the MDD of C, P(1), and P(2), while the regression methods of Gross and Hasund, Bachmann, and Tränkmann et al. under-estimated the values. Bachmann's method appeared to give the most reliable results.