Adjacent-segment degeneration is a known consequence of spinal fusion. Factors associated with this, and its effect on results, have not been well identified. This study sought to determine factors associated with adjacent-segment degeneration and the effect this degeneration has on results after lumbar fusion and instrumentation. Forty-nine patients were retrospectively reviewed after lumbar fusion with instrumentation and analyzed for adjacent-segment degeneration. Adjacent-segment degeneration occurred in 17 (35%) patients and was associated with increasing patient age, the use of interbody fusion, and a worsening of clinical results with time. In older patients with stenosis, it was associated with previous surgery. This should be considered when choosing operative techniques in treating lumbar pathologic conditions, especially in older patients.