Disproportionate impairment of naming nouns versus verbs and the opposite pattern have been reported in cases of focal brain damage or degenerative disease, indicating that processing of nouns and verbs may rely on different brain regions. However, it has not been clear whether it is the spoken word forms or the meanings (or both) of nouns and verbs that depend on separate neural regions. We tested oral and written naming of nouns and verbs, matched in difficulty, in patients with nonfluent primary progressive aphasia (nonfluent PPA; n = 15), fluent primary progressive aphasia (fluent PPA; n = 7), and amyotrophic lateral sclerosis with frontotemporal dementia (ALS-FTD; n = 6). Patients with nonfluent PPA and ALS-FTD, both individually and as groups, were significantly more impaired on verb naming than on noun naming and significantly more impaired on oral naming than written naming. Patients with fluent PPA showed the opposite pattern for both word class and modality, significantly more impaired naming of nouns versus verbs and significantly more impaired written versus oral naming. Results indicate that separate regions of the brain are essential for access to oral and written word forms of verbs and nouns, and that these neural regions can be differentially damaged in separate forms of PPA.