Basis pontis lacunes cause contralateral but rarely ipsilateral ataxia. We explored this phenomenon with isotope tract tracing in the rhesus monkey. Labeled pontocerebellar fibers cross midline and disperse widely in the opposite hemipons before coalescing in the brachium pontis. This anatomical arrangement suggests that small pontine strokes spare sufficient decussating pontocerebellar fibers to prevent ipsilateral dysmetria, and that ipsilateral dysmetria after large pontine stroke represents a disconnection syndrome.