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The severity of spondylolisthesis is determined by the degree of slippage observed on X-rays. The Meyerding's Grading System is most commonly used by clinicians to measure the degree of slippage which include:

• Grade I = 1% to 25%
• Grade II = 26% to 50%
• Grade III = 51% to 75%
• Grade IV = 76% to 100%
• Grade V is called spondyloptosis and occurs when the L5 vertebra completely slides over the top of the sacrum.

The majority of cases of spondylolisthesis (up to 75%) are Grade I and approximately 20% are Grade II, both of which are considered low-grade slippage. Grade III and above is considered to be high-grade slippage. Another way of characterizing slippage is to consider Grades I and II (below 50% slip) as stable spondylolisthesis and Stage III and higher as unstable spondylolisthesis.

Slip progression after skeletal maturity is usually related to disk degeneration (secondary to arthritis) at the slip level. This is likely to develop during the fourth and fifth decades of life with the onset of degenerative spondylolisthesis. As the disk loses its structural and functional integrity, the lumbosacral junction (L5-S1) becomes unstable and the slip progresses and can cause severe incapacitating back and leg pain. This concurrent occurrence of disk degeneration and adult slip progression explains how asymptomatic spondylolisthesis can be present for at least two or three decades before becoming symptomatic.

In people with isthmic spondylolisthesis, slip progression is thought to occur in up to 30% of cases and takes place after the third decade of life. It is associated with signs of mechanical instability and spinal stenosis resulting in significant low back pain and possibly radicular (radiating) pain in the back of the legs. The magnitude of symptoms is not necessarily related to the grade of slippage since symptoms may be related to nerve compression which can happen with any grade.