Purpose: Previous studies have compared the use of interdental brushes and dental floss. However, none have attempted to compare their effects on subgingival plaque. Nor have smokers been excluded from previous studies, where they may have affected the assessment of gingival inflammation. Materials and Methods: The present study compared, in untreated patients suffering from mild to moderate periodontitis, the efficacy of dental floss (DF) and interdental brushes (IDB) in the reduction of plaque, gingival inflammation and probing depth in a one-month period prior to subgingival debridement. Ten patients used DF for one side of the dentition and IDB for the other side for one month. Oral hygiene instruction was given at baseline. Measurements were made at baseline and at one month. Results: With IDB, the mean approximal plaque score reduced supragingivally from 14.5 to 5.7 at one month, and with DF, from 12.9 to 5.3; subgingivally the score reduced from 17.3 to 6.7, and 16.7 to 8.1 respectively (p 0.001). BOP and mean probing depth reduced over time for IDB sites, but not DF sites (p 0.01). Overall there were no differences between the two devices. Patients preferred IDB because of its simpler method of use. Conclusions: The use of IDB and DF resulted in similar beneficial effects on subgingival plaque and proximal gingival health.