Three cases of atlanto-axial interspinous articulation (AIA) were reported. In a case, the articulating C1 posterior arch and C2 spinal process were removed en bloc during surgery. In an other case, articulating parts of C1 and C2 arches were excised by chisel. These had facets with glistening smooth surfaces and were consisted of fibrocartilage histologically. The facets were connected by a membrane containing the synovial layer, loose connective tissue layer with small vessels and the dense fibrous layer. With review of X-ray findings of present cases and cases reported in the literature, the AIA was classified into (1) simple type and (2) hyperplastic type. It was stressed that the hyperplastic type of AIA can produce definite cord compression.