SummaryIn the psychiatric treatment of patients with mild learning disabilities or borderline intellectual functioning, signs and symptoms of psychiatric disorders are sometimes misinterpreted as behaviour that reflects problems that are known to patients with mental retardation. We report on two case studies in which lithium therapy made a substantial contribution to (partial) recovery. One patient had bipolar disorder and the other had a major depressive disorder combined with suicidal behaviour. Each patient also had a mild learning disability or borderline intellectual functioning.