This investigation evaluated the efficacy of obtaining baseline culture and sensitivity data on a routine basis from the patient with adult periodontitis. Patients diagnosed with chronic adult periodontitis, rapidly progressive periodontitis, or refractory periodontitis were followed for up to 6 years. More than 95% of patients with chronic adult periodontitis were successfully treated with mechanical therapy alone. Approximately one half of the patients with rapidly progressive periodontitis were treated successfully without antibiotics. All of the patients with refractory periodontitis required systemic antibiotics as part of treatment. Most patients with chronic adult periodontitis exhibited one or two species of organisms at baseline, and these organisms were eliminated or reduced to low levels by mechanical therapy. In contrast, patients with rapidly progressive or refractory periodontitis consistently demonstrated multiple species and required systemic antibiotics in conjunction with mechanical therapy to alter the subgingival microbial milieu. Routine culturing and antibiotic therapy is contraindicated in patients with chronic adult periodontitis, but may be beneficial for successful treatment of patients with rapidly progressive or refractory periodontitis.