Type 2 diabetes is known to be associated with cognitive dysfunction and an increased risk of dementia in the elderly. Although it is one of the most efficacious therapies in diabetic patients, insulin therapy requires that patients learn to inject themselves with insulin. We studied the association between brain atrophy detected on magnetic resonance imaging (MRI) of the head and the ability of type 2 diabetic patients to learn self-injection. MRI of the head was performed in 41 type 2 diabetic patients aged 60-80 years old. The area of the cerebrum relative to the intracranial area (the brain parenchymal fraction: BPF) was calculated with the WinROOF software program. Learning ability was assessed by counting the number of training sessions needed to acquire the ability to self-inject. Patients were divided into a failure group and success group. The average age was significantly higher (P < 0.001) and the BPF was significantly lower (P < 0.001) in the failure group (patients requiring 14 or more training sessions) than the success group (the remaining patients). A binary logistic regression analysis revealed that both age and BPF were independent predictors of success/failure in acquiring self-injection ability (P = 0.005 and P = 0.031, respectively). We conclude that brain atrophy on MRI is an important determinant of the ability of patients to learn insulin self-injection.