Carcinoid tumors grow slowly and usually appear in the gastrointestinal tract or lung. Gastrointestinal symptoms include skin flushing and diarrhea, and symptoms that appear if the tumor is in the lung include difficulty breathing. Carcinoid tumors typically begin in the cells that produce hormones of various organs in the gastrointestinal tract like stomach or intestines, though they can develop in the pancreas or the testicles in men and the ovaries in women.