Aims: The effects of COVID-19 containment measures on health-related lifestyle have been both favourable and unfavourable for health. Factors predisposing to unfavourable changes are still poorly known. In this short communication, we aimed to examine which socioeconomic and health-related factors predicted unfavourable lifestyle changes based on data from the same individuals before (2017) the pandemic and during the second wave (2020) of the pandemic in Finland. Methods: This individual-level follow-up study was based on a nationally representative, two-stage stratified cluster sample of Finnish adults from the FinHealth 2017 Study, conducted in Spring 2017, and its follow-up survey, conducted in Autumn 2020. A total of 3834 men and women aged 25–69 years at baseline had information of selected lifestyle factors (vegetable consumption, leisure-time physical activity, sleeping problems and nightmares) available at both time points. Odds ratios and 95% confidence intervals for unfavourable lifestyle changes (yes/no) according to socioeconomic and health-related factors were calculated using logistic regression models taking into account the sampling design and non-response. Results: We found that those having poor health (i.e. psychological distress, poor self-rated health or chronic diseases) or disadvantaged socioeconomic background before the pandemic were prone to unfavourable lifestyle changes during the follow-up. Conclusions: Observed unfavourable lifestyle changes in vulnerable population groups may accelerate health inequalities. Targeted health promotion actions are needed to prevent this unfavourable development.