To further examine the previously suggested inverse relationship between Porphyromonas gingivalis and Prevotella intermedia in periodontal disease, 1016 samples taken from single or multiple (pooled) subgingival sites were cultured anaerobically and examined for the simultaneous occurrence of the microorganisms. P. gingivalis was isolated from 297 (29%) and Pr. intermedia from 501 (49%) samples. P. gingivalis was found as frequently with (14%) as without (15%) Pr. intermedia. The type of sampling had no effect on the occurrence of P. gingivalis with Pr. intermedia. However, female subjects harboured them in combination more frequently than male subjects. The mean proportions of P. gingivalis in the cultivable flora appeared to be lower when found with than without Pr. intermedia. Whether the detection of the combination, or P. gingivalis alone, has clinical relevance needs further clarification.