Increases in muscular cross-sectional area (CSA) occur in quadriplegics after training, but the effects of neuromuscular electrical stimulation (NMES) along with training are unknown. Thus, we addressed two questions: (1) Does NMES during treadmill gait training increase the quadriceps CSA in complete quadriplegics?; and (2) Is treadmill gait training alone enough to observe an increase in CSA? Fifteen quadriplegics were divided into gait (n = 8) and control (n = 7) groups. The gait group performed training with NMES for 6 months twice a week for 20 minutes each time. After 6 months of traditional therapy, the control group received the same gait training protocol but without NMES for an additional 6 months. Axial images of the thigh were acquired at the beginning of the study, at 6 months (for both groups), and at 12 months for the control group to determine the average quadriceps CSA. After 6 months, there was an increase of CSA in the gait group (from 49.8 ± 9.4 cm² to 57.3 ± 10.3 cm²), but not in the control group (from 43.6 ± 7.6 cm² to 41.8 ± 8.4 cm²). After another 6 months of gait without NMES in the control group, the CSA did not change (from 41.8 ± 8.4 cm² to 41.7 ± 7.9 cm²). The increase in quadriceps CSA after gait training in patients with chronic complete quadriplegia appears associated with NMES.