This study aims to describe factors associated with cognitive decline among 2584 subjects, aged 65-74, who were followed up for 54 months in the Medical Research Council Elderly Hypertension Trial (1982-1989). The subjects completed a cognitive test, the Paired Associate Learning Test (PALT), five times over this period. Decline on the PALT was associated with advanced age, male sex, rural residence, depression and low intelligence. These effects were modified by gender and level of pre-morbid intelligence. Advanced age, rural residence and number of cigarettes smoked daily were only associated with PALT decline among women of below median intelligence. The association between depression and PALT decline was only apparent in women of below median intelligence and men of above median intelligence. While these findings are consistent with other research into cognitive decline, they differ in some ways from reported risk factors for dementia, suggesting aetiological separateness. That women were more vulnerable than men to the effects of age and smoking raises the question of the impact on cognition of accelerated atherosclerosis after the menopause.