AIM: To investigate the association between ocular demodex folliculorum infestation and ocular surface manifestations in meibomian gland dysfunction (MGD). METHODS: Eight-six patients with MGD were enrolled. All enrolled subjects were tested in the following sequence: ocular surface disease index (OSDI), slit-lamp biomicroscope examination, corneal surface regularity index (SRI) and surface asymmetry index (SAI), tear fluid collection, fluorescein tear film break-up time (F-BUT), corneal fluorescein staining (CFS), Schirmer I test (SIT) and demodex folliculorum counting. Tear matrix metalloproteinase (MMP)-9 activity was assessed using MMP-9 activity assay kit. RESULTS: Among 86 MGD patients, 40 were positive for demodex. The ocular demodex-positive group showed significantly increased scores of OSDI (25.96±13.74 vs 18.07±11.55, P=0.01), lid margin abnormality (2.38±0.87 vs 1.98±0.91, P=0.04) and CFS (1.28±2.00 vs 0.94±1.36, P=0.01) compared to the ocular demodex-free group. The tear MMP-9 activity was higher in the ocular demodex-positive group (102.9±32.4 ng/mL) than the ocular demodex-free group (46.2±19.2 ng/mL, P=0.03). There was no significant difference in meibum quality and expressibility, SRI, SAI, F-BUT and SIT between the two groups (P>0.05 for each). No significant correlation was noted between the number of demodex and ocular surface parameters in demodex-positive MGD group (P>0.05 for each). CONCLUSION: Ocular demodex folliculorum infestation may be associated with ocular discomfort and ocular surface damage in MGD.