This investigation studied relative changes in periodontal conditions of 18 insulin-dependent diabetic patients. Measures of gingival inflammation, crevicular fluid aspartate aminotransferase (AST) levels, probing depth and attachment levels, the presence of three periodontal pathogens (Porphyromonas gingivalis, Bacteroides forsythus, and Actinobacillus actinomycetemcomitans) and serum antibody titers to these bacteria, and blood sugar levels (glycosylated hemoglobin, HbAlc) were studied before and 2 months after non-surgical debridement. Antibody titers to the same bacteria were also studied in sera from 18 sex- and age-matched periodontally healthy and non-diabetic subjects. Periodontal conditions showed significant improvement. The mean probing depth at 4 of the worst sites selected in each patient decreased from 5.7 mm to 4.8 mm (p < 0.0001). The mean full width probing depth changed from 2.9 mm (s.d. +/- 0.2) to 2.5 mm (s.d. +/- 0.3). A mean gain of 0.4 mm attachment level was recorded (P < 0.0001). The mean AST value decreased from 1009 microIU to 518 microIU (P < 0.006). Minimal differences in mean glycosylated hemoglobin values (HbAlc) were noticed before and after treatment. A. actinomycetemcomitans was never detected. P. gingivalis was present at 7% of the sites both before and after treatment. B. forsythus was found at 29% of sites (50% of patients) before and at 36% of sites (61% of patients) after treatment. Positive associations were found between the presence of B. forsythus and AST values, gingival index, probing depth, and attachment level (P < 0.05). Baseline serum IgG titers to P. gingivalis were significantly lower in the patients with diabetes (9.5 ELISA units vs. 28.5 ELISA units in the healthy controls). IgG titers to B. forsythus did not differ between diabetic and non-diabetic subjects. No changes in IgG titers occurred after treatment. Clinical improvements after mechanical non-surgical therapy in patients with insulin-dependent diabetes mellitus were modest after 2 months. Treatment did not eliminate B. forsythus and P. gingivalis and did not affect IgG titer responses. More intense therapy, and longer follow-up times, may be necessary to see more pronounced clinical and systemic effects.