Interest in the role of polyunsaturated fatty acids (PUFAs), particularly long-chain (LC) omega-3 (n-3) PUFAs, in mental health is increasing. This review investigates whether n-3 PUFA levels are abnormal in people with three prevalent mental health problems - attention deficit hyperactivity disorder, depression, and dementia. Data sources included PubMed, Web of Science, and bibliographies of papers published in English that describe PUFA levels in the circulation of individuals who have these mental health conditions. Although abnormal blood PUFA levels were reported in a number of studies, weighted comparisons of PUFA status showed no significant differences overall between people with mental health problems and controls. Whether those with low n-3 PUFA status are likely to be more responsive to n-3 PUFA supplementation is not yet resolved. Further studies assessing PUFA levels and mental status with greater uniformity are required in order to clarify the relationship between LC n-3 PUFA status and mental health.