In this article national statistics from Swedish sickness funds are analysed from 1892 to 1954. The study is intended to provide better knowledge of changes in sickness claims, and to contribute to the discussion of morbidity trends. National statistics on sickness benefits are available from 1892, and are studied until the introduction of mandatory health insurance in 1955. Membership of Swedish sickness funds increased more than tenfold between 1900 and 1950. Just a small portion of members were women in 1900; however, women constituted the majority of members by 1941. There was a gender difference in morbidity, but this variation was not consistent over time. Before 1935 men had more but shorter sickness episodes; however, after 1935 women had significantly higher sickness rates. The law in 1931 meant that the recruitment of members was widened and compensation for longer periods of illness was made possible. The continuous increase in sick days can be partly explained by legislative changes; however, the increase in the period 1910–32 cannot. The study thereby highlights the importance of incorporating legislative changes into the interpretation. This increased understanding of trends in sick leave provides a sound basis for extending the study to the analysis of individual‐level data.