The antiplatelet effects of low-dose aspirin (LDA) vary between individuals. Here, we investigated the relationship between the incidence of LDA-induced mucosal injury, antiplatelet effects of LDA, and intragastric pH. We evaluated gastric injury severity and platelet function using the VerifyNow® System before and after administration of 100 mg aspirin for 7 days to 18 young healthy subjects (study 1). We investigated whether injury was correlated with platelet function and gastric juice pH in 45 patients with cardiovascular disease administered LDA daily (study 2). In study 1, platelet aggregation was attenuated by LDA to different degrees. Although 55.6% of subjects (10/18) developed gastric injury of modified Lanza score (MLS)≥ 3, no significant difference in platelet function was detected between the mild (n= 8, MLS: 0-2) and severe injury groups (n= 10, MLS: 3-5). In study 2, the severity of LDA-induced injury was associated with gastric juice pH, but not with antiplatelet effects of LDA. In contrast to gastric juice pH, the antiplatelet effect had no correlation with the severity of gastric mucosal injury. Monitoring gastric acidity, rather than platelet function, may be useful for predicting the risk of gastric injury during LDA treatment.