The incidence rates of primary nervous system neoplasms in 5 districts (large administrative units) of Western Poland with a relatively homogeneous health care system but some contrasts in economic activity, as well as in 224 communes (smallest administrative units) located within these 5 districts, with a total population of 3,097,000 were estimated on the appointed day (31.12.79). Testing the hypothesis, assuming a homogeneous distribution of nervous system neoplasms over the region under investigation, that the incidence rates were distributed as a Poisson variable has revealed a focal distribution of these tumours; in 15 communes the frequency was higher than 9.0 and deviated from the remainder in the 5% right tail of the distribution. The main conclusion from the presented results was that tumours of the nervous system of which 90% were intracranially located, may occur in clusters or foci, and this could prove to be of clinical as well as statistical import.