What illnesses cause ataxia and hyperreflexia?

Some answers: Illnesses that affect the cerebellum and corticospinal tract, or the dorsal columns and corticospinal tract can cause these findings. Stroke is probably the most common, MS is also a possible explanation, there are also others (trauma, infection, neoplastic disease, etc.)

Here are a few: May see with spinocerebellar degenerations, such a freidrich's ataxia, not unique to find in multiple sclerosis, might see with von hippel lindau syndrome, sometimes occurs with lupus, vasculitis, sjogren's. Occasionally a meningioma at the base of brain can present this way. Some brainstem strokes do this. Might go to wikepedia, or a site such as mayo clinic, as more information in depth there.

Many: A variety of white matter diseases such as leukodystrophies, degenerative spinocerebellar degenerations, carcinomatous meningitis, tumors and bleeding into cerebellum, multiple sclerosis, arteritis, and vasculitis, structural disorders, at base of brain and foramen magnum.