Purpose: Genetic modification technique is widely used in plants for improving crop yields and qualities, and reducing pesticide use. Zoysia japonica is one of the widely planted grasses for lawning. Pollenless herbicide-tolerant genetically modified (GM) grasses (JG21-MS1 and JG21-MS2) were developed under surveillance for possible biohazard. Grasses may cause allergic contact dermatitis with direct contact on human skin. Patch testing with allergens was adopted to compare the incidences of allergic contact dermati￢tis to GM grass leaves with those of wild-type grass. Methods: Patch testing with controls and leaves of wild-type and GM grasses was performed in individuals with informed consent. Results: Ninety-seven individuals (mean age, 32±8 years; 48% males) were enrolled. For nickel sulfate as a positive control, 38 sub￢jects (39.2%) were positive. For GM grasses, 10 (10.3%) showed positive patch test results and 11 (11.3%), for wild-type grass, the proportions of which were similar among the tested grasses. Conclusion: JG21-MS1 and JG21-MS2 are not more biohazardous than wild-type grass in the risk of allergic contact dermatitis.(Allergy Asthma Respir Dis 2015;3:134-138)