The literature is replete with suggestions that patients with neurofibromatosis (NF) in general and von Recklinghausen NF (NF-1) in specific often manifest mild mental retardation. However, intellectual performance in NF-1 has only rarely been the subject of study in a comparatively uniform population; it has been reported mainly in the setting of case histories. In the present material, representing a population-based cohort of Gothenburg, Sweden, it was possible to assess intelligence levels in 71 of the 74 patients originally identified [Samuelsson B, Samuelsson S: Neurofibromatosis 1989;2:6-22]. Thirty-two of the seventy-one patients (45%) showed slight mental retardation and 2 of these were somewhat more retarded than the remainder. This slight mental retardation was usually recognized during the early school years; it did not appear to be progressive. While patients affected with this problem were placed in remedial or special classes in school, these patients generally did not acquire any vocational training. There is reason to suspect that NF-1 itself leads to some impairment of intellectual development in all patients, for the intellectual achievements of those whose intelligence was judged to be within the normal range appeared to fall short of the average. Non of the patients with normal intelligence had passed any academic examination and many showed achievements less than expected based on the achievements of their healthy relatives. In certain families, the NF appeared to be associated with mild mental retardation more than in other families. Mental retardation of a more severe degree, corresponding to special school level or lower, is not typical of NF-1.(ABSTRACT TRUNCATED AT 250 WORDS)