The purpose of this study was to determine the occurence of spirochetes and Wolinella sp. in untreated, nonselected subjects and to relate the presence of these organisms to changes in probing depth over a period of 180 days. Clinical parameters were recorded and subgingival microbial samples were taken mesiobuccally on the upper first molars of 120 subjects (mean age 37 +/- 14 years) with a history of no regular dental care. The number of motile rods and spirochetes were determined in the darkfield microscope. The samples were cultured anaerobically on a selective medium which favoured the growth of Wolinella sp. The grown microorganisms were submitted to a number of tests for the identification of W. recta. Of the 172 sites investigated longitudinally, 73 harboured motile rods, 45 harboured W. recta and 88 harboured spirochetes. 13 sites showed an increase in probing depth of greater than or equal to 2 mm. A multiple regresssion analysis revealed no statistically significant correlation between the numbers of W. recta at baseline and the likelihood of breakdown during the observation period. The number of spirochetes at baseline, however, was significantly related to the changes of probing depth (p greater than 0.001). It was concluded that spirochetes were more closely related to changes of probing depth in this population than the number of W. recta organisms present.