We encountered five otherwise healthy adults with alterations of the tarsal navicular bone compatible with spontaneous osteonecrosis. Four women had bilateral involvement and one man had unilateral involvement. The patients were 23-71 years old. The disorder was initially described by Mueller and Weiss and should not be confused with Koehler disease (osteochondrosis of the tarsal navicular in children). This group of patients was compared with five other patients (29-74 years old) with similar radiographic and clinical changes in whom an underlying disease (rheumatoid arthritis, renal failure, trauma, and lupus erythematosus) associated with osteonecrosis was known. Routine radiography in both groups defined characteristic abnormalities of the navicular bone (decreased size, a comma-shaped configuration, increased radiodensity, fragmentation, and medial or medial and dorsal osseous protrusion). MR in three patients confirmed alterations consistent with osteonecrosis. Three of the patients without underlying disease had bilateral involvement on plain films, with flat feet and hindfoot valgus deformity, leading to local pain and deformity. In a fourth patient, bilateral distribution was documented by MR as marrow alterations. Although no certain pathogenic explanation for spontaneous osteonecrosis of the tarsal navicular is known, trauma and chronic stress changes caused by physiologic pressure on the medial longitudinal arch in hindfoot valgus and increased tension forces of the plantar aponeurosis during weight-bearing (in pes planus) may be important. Discrimination of primary from secondary osteonecrosis of the tarsal navicular bone is not possible by radiologic means alone, although bilateral distribution, particularly in women, favors the diagnosis of spontaneous disease.