The standard established by the World Health Organization (WHO) to rate tumors.

• Grade I tumors are slow-growing, nonmalignant, and associated with long-term survival.
• Grade II tumors are relatively slow-growing but sometimes recur as higher grade tumors. They can be nonmalignant or malignant.
• Grade III tumors are malignant and often recur as higher grade tumors.
• Grade IV tumors reproduce rapidly and are very aggressive malignant tumors.