This study compared relative attachment levels (RAL) and probable crevice depths (PCD) from 6 periodontal probes (1 third, 4 second, and 1 first generation). A single clinician recorded RAL and PCD in 6 patients with chronic adult periodontitis (mean age 48.2 years) during two visits (interval: 1 week) using a Latin square examination order. Replicate measurements were recorded at the 4 interproximal sites of the Ramford index teeth to examine intra-probe differences. Additional single measurements were recorded at similar sites of units 11, 26, 31 and 46 to investigate interprobe differences. Intra-probe RAL and PCD reproducibility was < or = 1 mm at > 94% of the sites. All intra-probe Pearson correlation coefficients were > 0.85 (P = 0.00). Inter-probe RAL and PCD agreement was < or = 1 mm at > 49% of the sites for RAL and > 61% of the sites for PCD. All inter-probe Pearson correlation coefficients were > 0.42 (P = 0.00) for RAL and > 0.49 (P = 0.00) for PCD. Analyses of variance showed significant differences in RAL and PCD between the first generation probe and the second generation probes (P < 0.005); in RAL between the third generation probe and the first and second generation probes (P = 0.0354); and in PCD between the third generation probe and the first and second generation probes (P = 0.0475). Inter-probe differences were clinically significant in the recorded percentages of pockets > or = 4 mm and > or = 6 mm. Significant inter-probe differences were found in RAL and PCD which have clinical importance in data interpretation and comparison.