Adult T-cell leukemia (ATL) is an aggressive lymphoid proliferation associated with the human T-lymphotropic virus type I (HTLV-I). The intraocular invasion of ATL is a rare event. A 75-year-old man without any systemic disease underwent uneventful cataract surgery of the right eye. On postoperative day 6, the patient presented with blurred vision due to severe vitreous opacity in the right eye. Analysis of the vitreous fluid revealed a suspected ATL infection based on the flow cytometric analysis. Moreover, cytological examination of the vitreous specimen revealed flower cell infiltration, and HTLV-1 DNA was detected by PCR analysis of the vitreous sample. Monoclonal T-cell receptor chain rearrangement was also detected by PCR. Thorough analysis of a vitreous sample is essential for vitrectomy in vitreous opacity of unknown cause. Flow cytometric, cytological, and PCR analysis of vitreous samples is beneficial for determining the cause of this kind of severe illness.