The therapeutic effects of probiotics in patients with traumatic brain injury (TBI) remain unclear. This study aimed to investigate the effects of probiotic supplementation on cell adhesion molecules (CAMs), oxidative stress and antioxidant parameters in TBI patients. This randomised, double-blind, placebo-controlled trial included forty-six TBI patients who were randomly assigned to receive either a probiotic supplement (n 23) or a placebo (n 23) for 14 d. The probiotic capsule contained four strains of Lactobacillus (L. casei, L. bulgaricus, L. rhamnosus, L. acidophilus), two strains of Bifidobacterium (B. longum, B. breve) and Streptococcus thermophilus. Serum levels of intercellular adhesion molecule (ICAM)-1, vascular cell adhesion molecule (VCAM)-1, pro-oxidant–antioxidant balance (PAB), malondialdehyde (MDA), nitric oxide (NO), total antioxidant capacity (TAC) and arylesterase (ARE) activity were measured at the beginning and end of the trial. Dietary intakes of patients were also recorded at the beginning and end of the trial. At the end of the study, there were no significant changes in ICAM-1, VCAM-1, PAB, MDA, NO, TAC and ARE levels. However, patients who received probiotic supplements had significantly increased dietary intakes of energy, macronutrients, vitamin E, Zn, Cu and Se compared with the placebo group. This study provides evidence that probiotic supplementation for 14 d in TBI patients has beneficial effects on dietary intake. However, it did not affect serum levels of CAMs, oxidative stress or antioxidant parameters. These findings should be considered preliminary, and further research is needed to evaluate long-term and clinical outcomes.