16 advanced periodontitis patients were subjected to initial periodontal treatment and monitored every 3rd month during 42 months. Clinical characteristics at baseline and during the 42-month maintenance period were investigated for their association with probing attachment loss over the 42-month period, both on a patient level and on a site level. On a patient level, averaged full-mouth plaque and bleeding on probing scores over the maintenance interval showed little association with probing attachment loss. Little association was also observed for % sites with depth > or = 6 mm at baseline. However, a notable relationship was seen for % sites > or = 6 mm at 3 months. This finding initiated a separation of the 16 subjects into 2 groups based upon % sites > or = 6 mm at 3 months (groups 'high' and 'low'). Site level analyses for these groups showed little association between frequent presence of plaque at the sites over the maintenance interval and probing attachment loss. Frequent bleeding on probing showed limited relationship with attachment loss for group 'low', but an appreciable association for group 'high'. The findings suggest that advanced periodontitis patients with multiple residual probing depths > or = 6 mm at re-evaluation run a greater risk of developing sites with additional attachment loss than patients with few such residual depths. For such higher risk patients, bleeding on probing at maintenance examinations may be a useful indicator of subsequent deterioration at a site level.