BACKGROUND/AIM: Neurofibromatosis type 1 (NF1) is a genetic disorder with an incidence of approximately one in 3,000. More than half of the patients have new de novo pathogenic variants of the NF1 gene. In most family cases, all family members share an identical NF1-variant. The aim of the study was to investigate the very rare phenomenon of de novo variants in cases of familial neurofibromatosis type 1 and highlight its implications for genetic testing and counseling. PATIENTS AND METHODS: Patients underwent clinical examination in our NF outpatient clinic and genetic testing for the NF1-gene was performed by targeted sequencing. All family members were profiled by short-tandem repeat marker analysis. Additionally, a probability calculation was performed for this extremely rare event. RESULTS: In one NF1 family consisting of mother, father, and two sons, two different pathogenic variants of the NF1 gene were found. The father and one son share one NF1-variant and the other son carries a different de novo NF1-variant. Neither of these two NF1-variants was found in the unaffected mother. Short-tandem repeat analysis confirmed the paternity and revealed that the two sons inherited two different NF1-alleles from their father. The probability of two different NF1-variants occurring in one family is calculated as 1:9,000,000. CONCLUSION: Two different NF1-variants in one family is an extremely rare phenomenon: yet its occurrence is not impossible and therefore should be considered in genetic diagnosis and counselling. For an offspring with the indication for neurofibromatosis type 1, but lacking the familial pathogenic variant, a screening of the whole NF1-gene is necessary to detect potential new pathogenic variants and for exact diagnosis.