Synovial cysts of the cervical spine occur infrequently in the spinal canal and are most often associated with degenerative facet joints. Despite the prevalence of degenerative spinal disease, symptomatic synovial cysts are extremely uncommon. We report a rare case that showed an exacerbation of a cervical radiculopathy due to an acute expansion of the synovial cyst. Magnetic resonance (MR) images originally revealed a small cystic extradural lesion when the patient presented with neck pain and slightly numbness in the right hand. The patient's complaints subsequently subsided after administration of pain killers. However, 2 weeks after this, the patient experienced a spontaneous, sudden, severe radiating pain into the right arm without any accompanying cervical injury. MR images showed that the cyst had become markedly increased in size in the intervening 4 weeks and compressed the spinal cord laterally. Because the arm pain was so severe and neurologic examinations revealed the paralysis of the C8 nerve root, the synovial cyst was excised surgically and a good clinical outcome achieved. Thus, even if symptoms are mild and the size of the synovial cyst is small, acute expansion of the cyst might be rarely observed and careful management, including surgical consideration, is needed.