Murine progressive ankylosis was studied by light and electron microscopy (EM). By light microscopy, the earliest synovial lesion was inflammatory followed by synovial proliferation and cartilage erosions. Later changes included progressive joint ankylosis by fibrosis and ossification of articular and periarticular tissues and new bone proliferation. By EM, synovial cell proliferation and hypertrophy with collagen deposition was seen before the appearance of intracellular and extracellular hydroxyapatite crystals. These pathologic changes are the same as those found in the human spondyloarthropathies. Our EM findings demonstrate that hydroxyapatite crystals appear in the joints of ank/ank mice after the initial inflammatory phase.