BACKGROUND The most common cause of a sudden onset of painful proptosis with diplopia in an otherwise healthy adult is orbital pseudotumor. However, there are other conditions that mimic this presentation and must be ruled out with laboratory testing and imaging studies. METHODS A 41-year-old Hawaiian man sought treatment for an acute, progressive, painful, left ophthalmoplegia without exophthalmos. During the next week, a loss of accommodation and associated pupillary reaction, decreased visual acuity, color vision deficits, and a paracentral scotoma developed. CT and MRI revealed a mass in the orbital apex. All systemic findings were negative, and high-dose systemic steroid therapy was initiated. Symptoms resolved within hours of the first dose, and signs were completely absent 1 month later. Follow-up MRI revealed complete absence of the previously noted mass. CONCLUSIONS This is an atypical case of orbital pseudotumor, since there was no exophthalmos. It was diagnosed by clinical presentation, laboratory and imaging studies, and response to therapy.