CONTEXT: Chronic pain is often present in patients with diabetes mellitus. OBJECTIVE: To assess the effects of osteopathic manual treatment (OMT) in patients with diabetes mellitus and comorbid chronic low back pain (LBP). DESIGN: Randomized, double-blind, sham-controlled, 2×2 factorial trial, including OMT and ultrasound therapy (UST) interventions. SETTING: University-based study in Dallas-Fort Worth, Texas. PATIENTS: A subgroup of 34 patients (7%) with diabetes mellitus within 455 adult patients with nonspecific chronic LBP enrolled in the OSTEOPAThic Health outcomes In Chronic low back pain (OSTEOPATHIC) Trial. MAIN STUDY MEASURES: The Outpatient Osteopathic SOAP Note Form was used to measure somatic dysfunction at baseline. A 100-mm visual analog scale was used to measure LBP severity over 12 weeks from randomization to study exit. Paired serum concentrations of tumor-necrosis factor (TNF)-alpha obtained at baseline and study exit were available for 6 subgroup patients. RESULTS: Key osteopathic lesions were observed in 27 patients (79%) with diabetes mellitus vs 243 patients (58%) without diabetes mellitus (P=.01). The reduction in LBP severity over 12 weeks was significantly greater in 19 patients with diabetes mellitus who received OMT than in 15 patients with diabetes mellitus who received sham OMT (mean between-group difference in changes in the visual analog scale pain score, -17 mm; 95% confidence interval [CI], -32 mm to -1 mm; P=.04). This difference was clinically relevant (Cohen d=0.7). A corresponding significantly greater reduction in TNF-alpha serum concentration was noted in patients with diabetes mellitus who received OMT, compared with those who received sham OMT (mean between-group difference, -6.6 pg/mL; 95% CI, -12.4 to -0.8 pg/mL; P=.03). This reduction was also clinically relevant (Cohen d=2.7). No significant changes in LBP severity or TNF-alpha serum concentration were associated with UST during the 12-week period. CONCLUSION: Severe somatic dysfunction was present significantly more often in patients with diabetes mellitus than in patients without diabetes mellitus. Patients with diabetes mellitus who received OMT had significant reductions in LBP severity during the 12-week period. Decreased circulating levels of TNF-alpha may represent a possible mechanism for OMT effects in patients with diabetes mellitus. A larger clinical trial of patients with diabetes mellitus and comorbid chronic LBP is warranted to more definitively assess the efficacy and mechanisms of action of OMT in this population.