The involvement of extra-ocular muscles in malignant diseases has been described only sporadically. The purpose of this study was to estimate the prevalence of orbital muscle lymphoma and metastases and to analyse their radiological findings. In the time period from January 2000 to January 2010, 11 patients with extra-ocular muscle malignancies (EOMM) were retrospectively identified in the radiological database of our institution. There were four women and seven men with a median age of 58 years (range, 47 to 72 years). In three patients non-Hodgkin lymphoma (NHL), in seven cases intramuscular metastases of solid tumours and in one patient plasmacytoma of orbital muscles were diagnosed. In all, magnetic resonance imaging (MRI) was performed on 11 patients using a 1.5-T MRI scanner (Magnetom Vision Sonata Upgrade, Siemens, Germany). The diagnosis of EOMM was confirmed histopathologically by muscle biopsy in all cases. The prevalence of orbital muscle involvement in plasmacytoma was 0.3%, in NHL 0.4% and in carcinomas 0.1%. Clinically, EOMM presented as painless proptosis and motility disturbance. Medial and lateral rectus muscles were involved in most patients. On T2-weighted images, the lesions were isointense or mixed iso-to-hyperintense in comparison to the unaffected musculature. On T1-weighted images, all tumours were homogeneously isointense. After intravenous administration of contrast medium, most lesions showed moderate heterogeneous enhancement. Lymphomas and metastases are rare lesions of the extra-ocular musculature with a prevalence below 0.5%. Their radiological and clinical signs are non-specific and include painless muscle enlargement or masses. They should be considered in the differential diagnosis of diseases of extra-ocular muscles.