Purpose: To evaluate the caries prevalence and experience in primary molars in Polish 6- to 7-year-old children using the Caries Assessment Spectrum and Treatment index. Materials and Methods: This cross-sectional study was carried out on a sample of 332 out of 468 invited children (140 6- and 192 7-year-olds). Cluster sampling was used to select the participating schools. Dental status was evaluated according to the criteria dedicated to CAST. The means of the individual CAST scores were assessed. Data were statistically analysed using the Mann-Whitney U-test (p 0.05). The percentage of children according to the highest CAST score was calculated. The mean values of the dmft index, based on three different caries thresholds - CAST codes 3 to 7 (d3-7t), 4 to 7 (d4-7t) and 5 to 7 (d5-7t) - were calculated. Results: The response rate was 70.9%. One-fifth of the participants showed a healthy, functional dentition (CAST codes 0-2). The highest percentage of children had at least one carious tooth with pulpal involvement (26.2%), followed by children having an obvious dentin caries lesion (20.8%). The means for CAST codes 3, 4 and 5 were 0.61 ± 1.0, 0.19 ± 0.53 and 1.23 ± 1.61, respectively. The means of molars with pulpal involvement was 0.83 ± 1.32 and with dental sepsis 0.07 ± 0.33. Apart from filled teeth, no age-dependent differences were observed. The d3-7mft, d4-7mft and d5-7mft were as follows: 5.18, 4.61 and 4.41, respectively. Conclusion: The surveyed Polish children showed high caries prevalence and severe caries experience in the primary molars. The CAST index was a useful tool for the epidemiological analysis of primary molars. Keywords: CAST, children, dental caries, epidemiology, primary molars