The article presents an attempt to analyse population changes and to measure the strength of the impact of factors causing these changes in former voivodeship (province) cities in Poland. In view of the ongoing processes of suburbanisation, the discussion also concerns the areas surrounding the city, i.e. those creating urban systems together with the city. These zones were delineated, calling them demographic influence zones, because only demographic factors were involved in defining them. The research was conducted in the period between1999–2015, and took into account the administrative reform of the country that degraded 31 cities from voivodeship (NUTS-2) capitals to poviat (LAU-1) cities. The main aim of the study was to find an answer to the question: do the directions and the strength of population changes confirm a hypothesis of the destructive impact of the loss of administrative function on settlement units. The results of the study only partially confirmed this hypothesis. Although a decrease in the population is overwhelmingly predominant in the city core, in the case of the demographic influence zone, it has already increased. Counting both parts together, it was found that in half of the cases there was a decline and in the other half a growth of the population.