OBJECTIVE: We describe population-level cognitive development in early middle-age and evaluate whether cardiovascular risk factors for late-onset dementia influence cognitive change in midlife. METHOD: The sample from the PATH Through Life (PATH) Project (N = 2,530; 40-44 years of age at baseline) was drawn randomly from the community, followed for 8 years, and assessed on cognitive function, health, and lifestyle risk factors at 4 yearly intervals. A summary risk score (PATHrisk) was computed for presence of smoking, hypertension, depression, high body mass index, diabetes, and insufficient physical activity. RESULTS: Decline in processing speed and reaction time (RT) and improvement in memory and verbal ability were observed. Higher PATHrisk score was associated with poorer performance on all cognitive tests, except for RT. Participants with higher PATHrisk scores had greater slowing on choice RT over 8 years. Education was associated with cognitive test performance and was weakly protective against slowing of RT. Individual risk factors, primarily diabetes, smoking, and depression, were associated with cognitive function, and smoking was associated with decline in simple RT. CONCLUSION: Reliable change in cognitive function was detected in midlife, and decline occurred primarily in measures of cognitive speed. It appears that in midlife, the overall burden of cardiovascular risk factors is more important than individual risk factors. Midlife is a critical period for identifying modifiable risk factors for dementia in late-life, and evaluation of burden of risk factors is indicated for developing strategies to prevent cognitive decline in ageing.