We describe eight patients with enlarging intramuscular hematoma who had no history of a bleeding tendency or trauma, other than for trivial incidental events of daily life. The hematoma gradually enlarged over 5-42 days and then diminished or disappeared within 3-5 months. This type of hematoma differs from both typical acute and chronic hematomas which were previously reported as a pseudotumor or a chronic expanding hematoma. The most important clinical task is to differentiate it from a malignant soft tissue tumor. A course in which the mass shrinks or disappears clinically, confirmed by magnetic resonance imaging findings, suggests the diagnosis. Laboratory tests, the bleeding time, and the blood coagulation time were normal in all patients, but the factor XIII concentration was slightly decreased. Depressed factor XIII concentrations may be an important etiologic factor in the genesis of this hematoma.