Introduction: More than million Polish people and 420 million individuals around the world have a vegetarian diet. Aim of the study: To evaluate the effects of vegetarian and vegan diets on gastrointestinal symptoms. Material and methods: The study included 1,209 individuals (886 from Poland and 323 from the United States of America). In the study population, 702 individuals declared following a vegetarian diet and 365 a vegan subtype of this diet. The control group included 142 individuals who had no dietary restrictions. All subjects completed a questionnaire composed of 31 questions. It was available online and was posted on vegetarian internet forums. Results: Before diet change, 60% of future vegans and 52% of future vegetarians experienced bloating and a sense of fullness; 32% and 25%, respectively, suffered from heartburn, nausea or vomiting; 44% and 35% had a tendency for constipation; 11% of subjects in both groups had a poor appetite and 3% of vegans and 2% of vegetarians had skin problems. After diet transition 84% of vegans and 74% of vegetarians reported health improvement (p = 0.002). Bloating and a sense of fullness decreased to 13% in vegans and 12% in vegetarians (p < 0.001), heartburn, nausea and vomiting to 2% and 4%, respectively (p < 0.001), poor appetite to 2% and 3% (p = 0.001), and bowel movement problems to 8% and 4% (p = 0.001). The rate of skin problems rose to 10% in vegans and 16% in vegetarians (p = 0.001). Conclusions: After the introduction of a vegetarian or vegan diet in the lives of the subjects a significant decrease in gastrointestinal complaints was observed; however, this was accompanied by an increase in the rate of skin problems.