Cancer prevalence is the proportion of individuals in a population who at some stage during their lifetime have been diagnosed with cancer, irrespective of the date of diagnosis. Cancer prevalence statistics have generally been provided by a limited number of well established cancer registries that have been in existence for several decades. The advent of systematic follow-up of life status of incident cases and the availability of new statistical methodologies, now makes it possible for registries established during the 1970s or 1980s to provide prevalence data. The main problems encountered in the estimation of prevalence are the inclusion of: (i) cases lost to follow-up; (ii) cases known only from their death certificate; (iii) cases diagnosed before the start of registration; and (iv) the treatment of multiple tumours and migrations. The main aim of this paper was to review these problems and discuss, through the experience gained with EUROPREVAL, how they can be overcome. A method is presented for the calculation of prevalence of all cancers combined in the populations covered by the 45 cancer registries participating in EUROPREVAL. Prevalence of cancer is estimated to be 2% on average, with the highest values (3%) in Sweden and the lowest in Eastern Europe, with a minimum of approximately 1% in Poland.