National Accounts provide detailed information on the development of the economy detailed by sector of industry. However, a disaggregation by enterprise size class is not available. The available data on the size class structure of the economy shows huge and unrealistic fluctuations and can therefore not directly be used to disaggregate the information from National Accounts. This paper reviews some methods to smooth developments shown in source data with respect to the share of small, medium-sized and large enterprises. It appears that in principle, the Hodrick-Prescott filter is most suited to fulfill this task. A modified Hodrick-Prescott filter is used. In particular, the simultaneous smoothing of interrelated series, taking into account the definitional relations existing between them, performs quite well. The methodology has been tested for two rather different sectors of industry, i.e. chemical industry (large-scaled, business-to-business oriented, capital-intensive) and retail trade (small-scaled, oriented towards consumers, labor-intensive). It appears that adjusted series, according to industry experts, give a more realistic description of historical developments than the original series do.