Prolonged coughing, bronchitis or other chest infections, and bronchiectasis can cause blood in phlegm, according to WebMD. Additional causes include tuberculosis, lung cancer and other cancers of the respiratory system.
Sometimes a bit of blood in the phlegm is benign, especially if the patient is young and otherwise healthy, states WebMD. Bloody phlegm is potentially a serious cause for concern if a patient has a concurrent fever or finds blood in his urine or stool. Other symptoms that make coughing up blood problematic are chest pain, lightheadedness or difficulty breathing.
Bronchitis is caused by an inflammation of the bronchii, or air tubes in the lungs. The most common symptoms are coughing up grayish-yellow mucous, wheezing and sore throat. Bronchiectasis is a condition where the bronchial tubes become flabby, according to the National Heart, Lung and Blood Institute. They can no longer clear out mucus very well, which puts the lungs at risk for infection.
Tuberculosis causes patients to cough up bloody phlegm. Other symptoms are persistent, painful cough, night sweats, chills, fever, chest pain, fatigue, unexplained weight loss and general malaise.
Symptoms of lung cancer are similar to those of tuberculosis, but can also include hoarseness and wheezing, headache and pain in the bones, according to Mayo Clinic.