Necropsy studies suggest that thiamine deficiency is underdiagnosed in life, in part because the classical clinical presentations are uncommon. Anecdotal reports suggest that thiamine deficiency may contribute to the development of delirium, heart failure and peripheral neuropathy in elderly patients, but little systematic research has been reported. We examined thiamine levels in 36 consecutive non-demented, community-dwelling patients admitted to an acute geriatric unit. Marginal thiamine deficiency [thiamine pyrophosphate effect (TPPE) 15-24%] was present in 11 (31%) and definite thiamine deficiency (TPPE > 25%) in 6 (17%) patients. Delirium occurred in 6/19 (32%) patients with normal thiamine status and 13/17 (76%) thiamine-deficient patients (p < 0.025, chi 2 test). One or more other possible causes for delirium were present in all cases. One patient had ocular signs and a dramatic clinical response to vitamin B complex therapy. Absent ankle jerks were noted in 2/19 (10%) patients with normal thiamine status and 7/17 (41%) patients with thiamine deficiency (p = 0.06). There was no difference in anthropometric indices or in the prevalence of other nutrient deficiencies between the two groups. Thiamine deficiency is common in elderly patients admitted to hospital and may contribute to the development of delirium.