Objective: To test the efficacy of 4-aminopyridine (4-AP) on functional status, walking speed and vibration perception in patients with chronic, incomplete spinal cord injury. Methods: Twenty SCI patients were randomized in a trial with a double-blind, crossover design to receive four weeks of orally administered 4-AP, followed by a two-week wash-out period and four weeks of placebo, or vice versa. The total daily dose of 4-AP during the four weeks of treatment was systematically increased to a maximum of 0.5 mg/kg body weight. Evaluation of (side-)effects took place at the beginning, after one week, and at the end of each four-week study period. Results: No significant benefit was found on functional status (COOP-WONCA). A statistically significant treatment effect was found on the vibration perception threshold (VPT) in the left fingers, during the first study period. On average, patients receiving 4-AP treatment responded less favourably (mean increase in VPT of (0.29 (0.31) μm) than patients receiving placebo (mean decrease in VPT of (0.05 (0.35) μm) (p = 0.04). Neither comfortable nor maximum walking speed altered significantly following 4-AP treatment. Conclusions: No statistically significant, functional benefit from 4-AP was found for patients in the present study. Furthermore, no support was found for the possibility that an a priory selection of responsive patients would have yielded more favourable results.