Background: Physical activity (PA) has the potential to attenuate cardiovascular disease risk in midlife women through multiple pathways, including improving lipid profiles. Longitudinal patterns of PA and blood lipid levels have not been studied in midlife women. Our study identified trajectories of PA and blood lipids across midlife and characterized the associations between these trajectories. Methods: We evaluated 2,789 participants from the Study of Women's Health Across the Nation (SWAN), a longitudinal cohort study with follow-up over the menopause transition. Women reported PA using the Kaiser Physical Activity Survey at seven study visits across 17 years of follow-up. Serum high-density lipoprotein (HDL) cholesterol, low-density lipoprotein (LDL) cholesterol, and triglycerides were measured at eight study visits across the same 17-year follow-up period. We used group-based trajectory models to characterize trajectories of PA and blood lipids over midlife and dual trajectory models to determine the association between PA and blood lipid trajectories adjusted for race/ethnicity, body mass index category, smoking, and lipid-lowering medication use. Results: Women were 46 years old, on average, at study entry. Forty-nine percent were non-Hispanic white; 32 % were Black; 10 % were Japanese; and 9 % were Chinese. We identified four PA trajectories, three HDL cholesterol trajectories, four LDL cholesterol trajectories, and two triglyceride trajectories. The most frequently occurring trajectories were the consistently low PA trajectory (69 % of women), the low HDL cholesterol trajectory (43 % of women), the consistently moderate LDL cholesterol trajectory (45 % of women), and the consistently low triglycerides trajectory (90 % of women). In dual trajectory analyses, no clear associations were observed between PA trajectories and HDL cholesterol, LDL cholesterol, or triglycerides trajectories. Conclusions: The most frequently observed trajectories across midlife were characterized by low physical activity, low HDL cholesterol, moderate LDL cholesterol, and low triglycerides. Despite the absence of an association between long-term trajectories of PA and blood lipids in this study, a large body of evidence has established the importance of clinical and public health messaging and interventions targeted at midlife women to promote regular and sustained PA during midlife to achieve other cardiovascular and metabolic benefits.