Pain is the most disturbing symptom of diabetic peripheral neuropathy. As many as 45% of patients with diabetes mellitus develop peripheral neuropathies. To evaluate the effect of gabapentin monotherapy on pain associated with diabetic peripheral neuropathy. Randomized, double-blind, placebo-controlled, 8-week trial conducted between July 1996 and March 1997. Outpatient clinics at 20 sites. The 165 patients enrolled had a 1- to 5-year history of pain attributed to diabetic neuropathy and a minimum 40-mm pain score on the Short-Form McGill Pain Questionnaire visual analogue scale. Gabapentin (titrated from 900 to 3600 mg/d or maximum tolerated dosage) or placebo. The primary efficacy measure was daily pain severity as measured on an 11-point Likert scale (0, no pain; 10, worst possible pain). Secondary measures included sleep interference scores, the Short-Form McGill Pain Questionnaire scores, Patient Global Impression of Change and Clinical Global Impression of Change, the Short Form-36 Quality of Life Questionnaire scores, and the Profile of Mood States results. Eighty-four patients received gabapentin and 70 (83%) completed the study; 81 received placebo and 65 (80%) completed the study. By intent-to-treat analysis, gabapentin-treated patients' mean daily pain score at the study end point (baseline, 6.4; end point, 3.9; n = 82) was significantly lower (P<.001) compared with the placebo-treated patients' end-point score (baseline, 6.5; end point, 5.1; n = 80). All secondary outcome measures of pain were significantly better in the gabapentin group than in the placebo group. Additional statistically significant differences favoring gabapentin treatment were observed in measures of quality of life (Short Form-36 Quality of Life Questionnaire and Profile of Mood States). Adverse events experienced significantly more frequently in the gabapentin group were dizziness (20 [24%] in the gabapentin group vs 4 [4.9%] in the control group; P<.001) and somnolence (19 [23%] in the gabapentin group vs 5 [6%] in the control group; P = .003). Confusion was also more frequent in the gabapentin group (7 [8%] vs 1 [1.2%]; P = .06). Gabapentin monotherapy appears to be efficacious for the treatment of pain and sleep interference associated with diabetic peripheral neuropathy and exhibits positive effects on mood and quality of life.