In a real-time polymerase chain reaction-based assay with a Dentoflor kit Prevotella intermedia, a periodontal pathogen, was found to be present in 55 % of patients with chronic periodontitis. This percantage is significantly lower than the corresponding statistic for Porphyromonas gingivalis, Tannerella forsuthensis and Treponema denticola. In addition, hypercolonization of periodontium by P. intermedia was detected in 7.7 % of patients in the control group. This suggests that P. intermedia hypercolonization is an important prognostic factor indicating the severity of periodontal disease. The abnormal increase in P. intermedia is rarely a result of a monoinfection. Compared to other “red complex” pathogens, this species is more likely to co-occur with P. gingivalis, T. forsythensis and T. denticola, which leads to the most severe forms of chronic periodontitis. Unlike other “red complex” pathogens, P. intermedia showed no significant difference in occurrence and the degree of periodontal damage in male and female individuals with equal colonization levels.