This phase 3 trial is being conducted at approximately 80 sites in the United States (U.S.) to investigate whether lacosamide (SPM 927) at different doses reduces pain in subjects with diabetic neuropathy. Approximately 455 subjects will be randomized to placebo or to one of three doses of lacosamide. To qualify for this trial, subjects with symptoms of painful distal diabetic neuropathy ranging in duration from 6 months to 5 years must have an average pain intensity of greater than or equal to 4 on an 11-point Likert scale (0-10 scale) during the 7 day period prior to the start of treatment.