Many patients suffer from vitamin B12 deficiency and are thus exposed to irreversible sequelae if diagnosis occurs at a late stage. This prospective study undertaken by eight practitioners over a period of 12 months concerns early diagnosis. Blood vitamin B12 levels were measured in 152 patients presenting macrocytosis detected by systematic MCV analysis at the time of a blood test, a neuropathy or a recent cognitive, affective and behavioural problem, and were found to be lowered (< or = 175 pmol/l) in 54 patients of whom 43 had undergone vitamin B12 test treatment for 6 months. Haematological, neurological and psychiatric evaluation was carried out before and after treatment, and a diagnosis of deficiency was recorded in 24 patients based on unequivocal response to therapy. Improvement was greatest haematologically in 12 patients, neurologically in 6 patients and psychiatrically in 6 other patients, with 4 patients showing a combination of all modes. These 24 patients (mean age 69 years) suffered from numerous pathologies which were liable to complicate diagnosis in some of them: neurological (46%), psychiatric (37%), chronic alcoholism (33%), folic acid deficiency (29%), and diabetes (17%). The only diagnostic element used as a criterion of deficiency was an extremely low level of vitamin B12 (< or = 75 pmol/l). Marked macrocytosis or a combination of haematological and neuropsychiatric signs are strong indicators, but only improvement under treatment allowed a diagnosis to be made in the majority of patients. Macrocytosis was, however, not present in 6 of the 12 neuropsychiatric patients. The study thus identified a high proportion of patients with vitamin B12 deficiency who additionally presented, in equal proportions, both haematological and neuropsychiatric symptoms. Neither the clinical examination nor the vitamin B12 level in general permit early diagnosis based on a high probability index and long-term follow-up. Simpler methods for early diagnosis are therefore needed.