The frequency of occurrence of the Fuld profile for cholinergic deficiency was investigated in two clinical populations: inpatients who had suffered traumatic brain injury and outpatients who carried a diagnosis of Parkinson's disease. The observed incidence of positive Fuld profiles was not significantly different in the two groups, 14% in the traumatic brain injury group and 24% in the Parkinson's disease group. These findings are consistent with recent reviews of the sensitivity and specificity of the Fuld profile in various clinical and nonclinical populations. The generally low sensitivity of the Fuld profile does not support its usefulness in the differential diagnosis of dementia. However, it may serve as an indicator of cholinergic deficiency, which could be used to select patients who would be likely to respond to cholinomimetic therapies.