Twenty-nine patients with pediatric orbital pseudotumor underwent a wide variety of diagnostic tests including biopsies. The following abnormalities were discovered: peripheral blood eosinophilia (9/29 patients); elevated ESR (17/27); elevated antinuclear antibody titres in the Tolosa-Hunt variant (2/2); hypercomplementemia (2/3); and mild CSF pleocytosis (2/6). Thyroid function tests were normal in nine patients so studied. B-mode ultrasonography performed on 12 patients displayed abnormalities in all cases (some combination of Tenonitis, myositis, perioptic inflammation, or mass effect). Computed tomography in seven patients provided higher resolution confirmation of these findings. Orbital bone changes and serious sinus disease were absent on routine radiographic studies. Biopsies performed on 16 patients disclosed mild lymphocytic inflammation in all cases, fibrosis and tissue eosinophilia in 9 biopsies (6 correlating with peripheral blood eosinophilia). Nine biopsies demonstrated a lipogranulomatous response to damaged fat cells. A true vasculitis or extensive lymphoid hyperplasia was not identified in any biopsy specimen.