OBJECTIVE The aim of this study was to analyse the proliferation rate of neurofibromas, in neurofibromatosis type 1 (NF1) patients in order to find out whether tumor growth can be correlated with the different subtypes, size and localisation of tumors, or gender. Large tumors and those localisations that do not allow a complete resection, e.g. the trigeminal branch plexiform neurofibromas, often require repeated surgical interventions. Therefore, the question whether partial resection is associated with alterations of the tumor type and proliferation is of great interest. METHODS We investigated 317 specimens of 96 patients. Twenty-five specimens were identified as local recurrences, all of them being consecutive resections in the previously operated area. All patients were NF 1-affected individuals who fulfilled the US National Institute of Health consensus criteria for defining the disease. The proliferation index (PI) was assessed on formalin-fixed, paraffin-embedded tissue stained with the MIB- 1 antibody (Ki-67 antigen). The PI was evaluated in three high-power fields (0.1 square millimeter) in the area with the highest proliferative activity. The correlations were calculated according to Spearman-Rho. RESULTS Men were more often surgically treated in the head and neck than women (p < 0.02). Plexiform neurofibromas were more frequently operated on in the head and neck than in other regions (p < 0.01). Older patients were more often treated for the diffuse cutaneous type of neurofibromas (p < 0.0001). The type of tumor did not differ from primaries to recurrent tumors. The MIB- 1 PI showed no association with any of the clinical parameters. In particular, there was no difference of the MIB-1 index between primaries and recurrent tumors. CONCLUSIONS This study showed, for the first time, that proliferation in neurofibromas is not enhanced in previously partially resected neurofibromas. Hence, the argument that trauma or surgery for neurofibromas might promote proliferation, especially in the plexiform neurofibroma, is not supported by the results of the present study. Further this analysis demonstrated interdependencies between tumor type, localisation, age and gender indicative of the social difficulties encountered by the NF1 patients which may be helpful for the advising practitioner.