Intraosseous, periodontal defects in 12 subjects initially treated by root planing alone (21 defects) or by flap surgery (21 defects) were monitored during a 5-year postoperative interval. Maintenance therapy during this interval was limited to reinforcement of oral hygiene and tooth polishing every 6 months. No subgingival instrumentation was performed at the defect sites. Longitudinal clinical measurements demonstrated that surgically-treated lesions responded with somewhat more reduction of probing depth and more gain of probing bone level than root-planed lesions. Mean gains of probing attachment level were similar for the 2 treatments. Some relapse of the clinical conditions could be observed towards the end of the 5-year observation interval compared to the results at year 1 and year 2. However, the majority of defects subjected to either treatment showed 60-month recordings of probing attachment and probing bone levels equal to or slightly improved compared to those at baseline. Counts from cultures of subgingival, microbial samples were obtained at 42, 48 and 60 months. No significant difference between the 2 therapies was observed for the investigated groups of micro-organisms.