Dyslipidaemia is a modifiable cardiovascular risk factor with prognostic implications. Current strategies for lipid management in young adults are largely based on expert recommendations. We investigated the risks of death and cardiovascular disease in relation to each lipid component to establish evidence for primary prevention in young adults. In this nationwide population-based cohort study, we analysed 5,688,055 statin-naïve subjects, aged 20-39 years, undergoing general health check-ups between 2009 and 2014. The endpoint was a composite of clinical events including death, myocardial infarction (MI), and stroke. We compared the incidence and risk of clinical events according to each lipid variable. During follow-up (median 7.1 years), clinical events occurred in 30,330 subjects (0.53%): 16,262 deaths (0.29%), 8578 MIs (0.15%), and 5967 strokes (0.10%). The risk of clinical events gradually increased with increasing total cholesterol (TC) and triglycerides and decreasing high-density lipoprotein cholesterol (HDL-C), largely driven by MI. Low-density lipoprotein cholesterol (LDL-C) had a J-shaped association with clinical events, showing the lowest risk for LDL-C of 84-101 mg/dL. Among lipid variables, triglycerides remained the sole independent predictor (adjusted hazard ratio, 1.20;  < 0.001) after adjusting for conventional risk factors. For statin-naïve young adults, the risk of clinical events was proportional to lipid levels, positively with TC and triglycerides, negatively with HDL-C, and J-shaped with LDL-C. Triglycerides had an independent and the strongest association with the clinical events. Screening and intervention for abnormal lipid levels, particularly triglycerides, from an early age might be of clinical value.