Membership in mutual health insurance societies spread among industrial workers in the late nineteenth century. We study determinants of such membership among male workers in Swedish manufacturing by using matched employer–employee data from three industries covering all workers (i.e. members and non-members, N > 12 000) and firms around 1900. We find remarkably high rates of membership overall, and especially among married workers. The association between marital status and health insurance suggests that selection into health insurance societies was ‘propitious’ rather than ‘adverse’. Many workers became members well before the age of 40 years, when their health began to deteriorate, and this coincided with the average age of first marriage for men, occurring in their late twenties. Being married and having membership was more marked in firms with voluntary membership and was important for the viability of the mix of voluntary and compulsory health insurance societies emerging in Nordic countries around 1900. Findings support the idea that health insurance can attract high levels of membership under voluntary schemes and suggest why it took so long before statutory health insurance covering sickness absence and workplace accidents was introduced in Sweden.