purpose To analyze the clinical, morphologic, immunophenotypic, and molecular genetic differences between idiopathic inflammatory pseudotumor and lymphoid tumors of the orbit. methods 209 patients with IOIP and 110 patients with lymphoid tumors seen between January 1, 1978 and December 31, 1999 in Zhongshan Ophthalmic Center, Sun Yat-sen University were evaluated retrospectively. results More patients with lymphoid tumors had palpable mass than patients with idiopathic orbital inflammatory pseudotumor (IOIP) (P < 0.0001), with the percentage of 90% and 65%, respectively, whereas more patients with IOIP had swollen eyelid, eyelid or conjunctival congestion, pain, retinal folds or hamorrhage, and optic nerve atrophy than patients with lymphoid tumors, with the percentage of 55% and 40% (P = 0.014), 42% and 24% (P = 0.001), 24% and 1% (P < 0.0001), 14% and 4% (P = 0.004), 7% and 2% (P = 0.043), respectively. Ultrasound and computed tomography/magnetic resonance image (CT/MRI) scan usually could not differentiate IOIP from lymphoma. Nearly one third of patients with IOIP could not be easily differentiated pathologically from lymphoid tumor. Immunophenotypic, and molecular genetic analyses can differentiate IOIP from lymphoid tumor based on polyclonal or monoclonal proliferation of lymphocytes with the IOIP being polyclonal and lymphoma monoclonal. conclusions Idiopathic orbital inflammatory pseudotumor and lymphoid tumor showed distinguishing clinical, morphologic, immunophenotypic, and molecular genetic characteristics.