The aim of this study was to assess the growth of incidental meningiomas, to establish a strategy for dealing with these tumours. The cases of 37 patients with a meningioma revealed incidentally by computerized tomography or magnetic resonance imaging, who were followed at least once by an additional imaging study, were reviewed. The tumour volume was calculated, to estimate the annual growth rate of the incidental meningiomas. Nine of the 37 patients (24.3%) showed a considerable increase (the annual growth rate > 1 cu cm/year) in their tumour volume (tumour growth). There was no significant difference in the follow-up period, age, or the volume of tumour between the patients with and without tumour growth. However, a multivariate analysis revealed that the likelihood of tumour growth independently and significantly increased according to a decrease in the age of the patients (Odds ratio 0.18 for one-standard-deviation change (ISD) 12.6 years, p = 0.042) and according to an increase in the volume of the tumour (Odds ratio 3.64 for ISD 4.46 cu cm, p = 0.042). The majority of patients with incidental meningioma can be apparently observed without any surgical intervention, because their annual growth rates are generally less than 1 cu cm/year. However, clinical and radiological observations would be advisable for these patients (especially young patients and patients with a large tumour), in view of the presence of rapidly growing tumours in some of the patients.