The neuropsychiatric phenotype associated with hyperprolinemia type I (HPI) is still under debate. To our knowledge, no long-term follow-up on patients with HPI has been reported so far. We have previously described the clinical, biochemical, and molecular features of four patients with HPI. Here, we report on the neuropsychiatric and genotype features of an expanded sample of 10 patients with HPI with a mean follow-up duration of 11 years. Epileptic manifestations and/or cognitive impairment were prevalent at onset, but they were subsequently replaced by psychiatric disorders. Social behavior and relational skills were considerably impaired in the majority of cases. Learning disability was present in one patient. The complex neurochemical effects of proline on the central nervous system and genotype/phenotype correlations were discussed.