Mental illness affected 23 (33%) of the 69 patients with von Recklinghausen neurofibromatosis (NF-1) who were the primary subjects of this report. Moderately severe or severe mental illness, that is, grade 2 and 3 according to the Comprehensive Psychopathological Rating Scale (CPRS), was found in 15 of the 69 patients (22%). The 23 mentally ill patients did not show any uniform clinical psychiatric syndrome. The most commonly occurring psychiatric diagnoses were depressive syndrome, anxiety state with vegetative dysfunction and organic brain syndrome. There was a significant positive relation between depressive syndrome and organic brain syndrome according to the CPRS rating. Half of the 69 patients complained of mental symptoms in the form of hostile feelings and autonomic disturbances and these were the most frequent psychiatric symptoms; they were also frequent among patients without mental illness. Other common symptoms were sleep difficulties, tiredness, and aches and pains. Among patients with NF-1 there was a significantly positive relation between mental illness and mental retardation, but no significant positive relation between mental illness and physical severity of NF-1. Patients with mental illness also showed a significantly increased frequency of positive neurological findings indicating central nervous manifestations of NF-1.