The aim of this study was to compare the periodontal probing depth in teeth exposed to periodontal treatment with teeth having received no treatment. In pockets deeper than 4 mm, a significant difference was found between the two groups. In the non-treated teeth, metal strips used for probing penetrated an average of 0.29 mm into the connective tissue apical to the junctional epithelium. The average probing depth for teeth exposed to scaling, root planing, curettage and chlorhexidine rinses for 1 month was 0.31 mm coronal of the connective tissue attachment within the junctional epithelium. The results indicate that care should be taken when evaluating changes in periodental probing measurements during longitudinal studies.