There are several discrepancies with respect to the composition of subgingival plaque in gingivitis and in inactive adult periodontitis (AP). In this study we compared subgingival plaque samples taken from gingivitis sites to those from inactive periodontitis sites of the same patients. Of 44 patients plaque samples from 86 gingivitis sites and 92 periodontitis sites were analysed. Darkfield microscopy showed a higher proportion of spirochetes and mobile rods in the periodontitis sites. Analysis of cultures revealed a higher and statistically significant number of anaerobes in the periodontitis sites (1.7 x 10(7) vs 3 x 10(6), p = 0.006). The following bacterial species were isolated more frequently from periodontitis sites than from gingivitis sites: Actinobacillus actinomycetemcomitans (18% vs 10%), as well as the black-pigmented Prevotella intermedia (68% vs 48%) and Porphyromonas gingivalis (48% vs 28%). On the one hand, these small differences in the bacteriological parameters can be explained by the fact that both gingivitis and periodontitis plaque samples were taken from the same periodontitis patients. An infection of the gingivitis sites from the parodontitis sites within the same patient could not be excluded. On the other hand, the mean probing depth of the gingivitis sites was relatively high, 3.6 mm (measuring point interdental plus pseudo-pocket) which may favor the growth of anaerobic bacteria.