Skip to main content

What are the 4 stages of pneumonia?

Medically reviewed by Leigh Ann Anderson, PharmD. Last updated on June 21, 2021.

Official answer


The 4 stages of untreated lobar pneumonia are:

  • Stage 1: Congestion
  • Stage 2: Red hepatization
  • Stage 3: Grey hepatization
  • Stage 4: Resolution

Pneumonia is an infection in your lungs. It is a serious infection in which the air sacs (alveoli) in your lungs fill with pus, blood cells and other liquid. Almost all cases of pneumonia are caused by viral or bacterial infections. Lobar pneumonia is an acute bacterial infection of the lung and affects one or more sections (lobes) of your lungs.

Stage 1 (congestion) occurs within 24 hours of infection. Many bacteria are present in the lungs but few white blood cells are available to fight the infection. The lungs may look red from increased blood flow and swelling of the lung tissue.

Stage 2 (red hepatization) occurs after 48 to 72 hours and lasts for about 2 to 4 days. The affected lung becomes more dry, granular and airless and resembles the consistency of liver. Red cells, white cells, bacteria and cellular debris can clog the lung airways.

Stage 3 (grey hepatization) occurs on day 4 to 6 and continues for 4 to 8 days. The lung looks grey or yellow in color but still has the consistency of liver. Fibrin, hemosiderin and red blood cells break down and lead to a more fluid-like exudate. Macrophages, a type of large white blood cell, start to form.

Stage 4 (resolution) is the final recovery stage and occurs during days 8 to 10. Fluids and breakdown products from cell destruction are reabsorbed. Macrophages (large white blood cells) are present and help to clear white blood cells (neutrophils) and leftover debris. You may cough up this debris. The airways and air sacs (alveoli) return to normal lung function. Any remaining lung swelling may lead to chronic lung disease (such as airway narrowing or pleural adhesions).

Pneumonia symptoms can include a cough with green, yellow or bloody phlegm or pus, chills, a fever, trouble breathing and shortness of breath, fatigue, chest pains and loss of appetite.

If you suspect you have contracted pneumonia, see your doctor promptly. Pneumonia can range from a mild illness to life-threatening complications. It is most serious for infants and young children, people older than age 65, and people with health problems or weakened immune systems. Vaccines can prevent some forms of pneumonia.

Learn more: Pneumonia Overview, Symptoms and Treatments


Related medical questions

Related support groups