Coughing up clear phlegm is commonly a sign of infection in the respiratory passageways, such as acute bronchitis, which tends to come on after a bad cold or flu, according to Phaa.com. The coughing up of clear phlegm may also be due to certain acute viral infections.
Although coughing up clear phlegm is less concerning than coughing up phlegm that is yellow, green or red in color, it should not be dismissed because it may still be a symptom of an underlying disease, advises Phaa.com. Certain noninfectious conditions, such as asthma or allergies, may also be the cause of excessive production and coughing up of clear phlegm. Individuals experiencing this symptom should seek the advice of a medical professional for a proper diagnosis.