One thousand one hundred and sixty-one non-polio enterovirus strains, isolated during regular screening of Finnish sewage specimens, were analysed for serotype distribution seasonally through 20 years, and the findings were compared with similar data based on 1681 clinical isolates. Coxsackievirus B4 (CBV-4), CBV-5, echovirus 11 (EV-11), EV-6, CBV-2 and CBV-3 were the most common serotypes in sewage, whilst CBV-5, EV-11, coxsackievirus A9, EV-22, CBV-3 and EV-30 were the most common clinical isolates. Reasons for the differences are not known but several explanations are possible. Seasonal variation of enterovirus occurrence in both sources showed an expected peak in the autumn with a trough in the spring. The occurrence of enteroviruses was closely correlated with monthly recordings of mean relative humidity. A further observation concerning the clinical specimens in Finland was the relative excess of some serotypes, such as echovirus 22 and coxsackievirus A9, and paucity of others, for instance, echoviruses 4 and 9, when compared to published data from other countries. This is consistent with the idea of geographically restricted circulation of enteroviruses.