Background: The sustainability of the Norwegian welfare state is being challenged by demographic changes. One consequence of increased life expectancy is that the working population is expected to work longer. Good mental and physical health is considered a key prerequisite for an extended working life. Previous studies have identified several associations between independent lifestyle risk factors, work ability and sick leave. However, few studies have investigated these associations using a summative lifestyle risk index. Aim: The aim of this thesis was to investigate potential associations between multiple lifestyle risk factors, work ability and sick leave in a general working population in Telemark County in south-eastern Norway. A further aim was to explore such associations within different non-communicable disease groups. Materials and method: At baseline of the longitudinal Telemark Study (2013), a selfadministered questionnaire was posted to 50 000 persons aged 16–50 in both rural and urban parts of Telemark County. Of these persons, 48 142 were eligible and a total of 16099 completed and returned the questionnaire. The questionnaire covered the following areas: personal information, working conditions, respiratory symptoms, respiratory symptoms and work, smoking and snuff habits, living conditions, childhood and family, physical activity and diet, and other diseases and illnesses. In 2018, a five-year follow-up questionnaire was sent to the 16 099 persons who had responded in 2013. In total, 7 952 persons completed both questionnaires (2013 and 2018). Logistic regression analysis was used to explore the associations between multiple lifestyle risk factors, work ability and sick leave. Other statistical analyses were also performed, including interaction analysis, correlation and association testing, and calculation of the population attributable fraction. Main results: All subjects included in Paper I and Paper II had worked in the preceding 12 months and answered the questions on work ability and lifestyle risk factors (n=10 355). In Paper I, individual lifestyle risk factors and a lifestyle risk index were associated with reduced work ability. This finding remained consistent after adjustment for age, sex, current occupation and education. In Paper II, physician-diagnosed asthma was found to be an effect modifier in the association between lifestyle risk factors (obesity, smoking and lifestyle risk index) and sick leave. Paper III included individuals who had been engaged in work in the preceding 12 months at both baseline and follow-up (n=6 267). The study found that unhealthy diet, low physical activity and smoking were associated with low work ability. This finding remained consistent after adjustment for potential confounders (Paper III). Further, the study showed that high body mass index, former and current smoking, and high and very high lifestyle risk indices were associated with higher rates of sick leave. Former smoking was associated with low work ability among persons who reported mental illness, while current smoking was associated with sick leave among persons who reported cardiovascular disease, diabetes or mental illness (Paper III). Conclusion: Based on these three papers, this thesis suggests that individual lifestyle risk factors may be linked to lower work ability and higher rates of sick leave. In particular, poor work ability was associated with a higher lifestyle risk index score. This was also shown for sick leave at follow-up. In all three studies, smoking was consistently associated with low work ability and increased sick leave (Papers I–III). Lastly, physician-diagnosed asthma was an effect modifier in the association between obesity, smoking and lifestyle risk index and sick leave (Paper II). These findings add to current knowledge and support the hypothesis that policies aimed at reducing lifestyle risk factors may benefit population health and extend working life. Also, as smoking is particularly linked to a social gradient in health, this finding warrants future attention. Future studies examining lifestyle risk factors may benefit from the inclusion of co-occurring lifestyle risk factors and the assessment of work measures (i.e. work ability and sick leave) as outcomes considering socioeconomic differences.