Pairs of posterior bitewing radiographs of forty children 13—15 years of age at the third and fourth examinations of three-year clinical trials were assessed by one examiner for primary caries. Duplicate assessments were carried out in independent sessions, recording simultaneously absence/presence of caries and caries degree, and in the same way for three different diagnostic levels recording only absence/presence of caries.Caries diagnostic inconsistency rate (D.I.R.) in per cent was always lower than 34 and usually below 20; a finding which is in agreement with previously published results. No preference could be expressed for any particular diagnostic level on the basis of the primary caries scores, but recording only relatively advanced lesions tended to give most favourable results for DFS increment scores.The recording of both absence/presence of caries and caries degree simultaneously had a small, but not consistently adverse effect on the D.I.R. and reliability coefficient.As there was no marked difference in examiner error associated with two of the demarcation lines used for recording of caries degree, i.e. half way through enamel and the A.D.J., the scoring codes described here may be suitable for studies of radiographic caries progression.