The fluorogenic enzyme assay for palmitoyl-protein thioesterase (PPT) has greatly facilitated the diagnosis of infantile neuronal ceroid lipofuscinosis (Santavuori-Haltia disease) and the search for possible new variants with atypical clinical presentation. Here, we present the first cases of adult neuronal ceroid lipofuscinosis with onset in the fourth decade of life due to a profound deficiency of PPT. The causative mutations in the CLN1 gene were the known, deleterious mutation R151X and the novel missense mutation G108R. Patients presented at onset (31 and 38 years), with psychiatric symptoms only. At present (ages 56 and 54 years), visual, verbal, and cognitive losses have progressed and both patients have cerebellar ataxia and cannot walk without support.