ABSTRACT.Sixty‐seven neuroleptic‐medicated mentally handicapped subjects in a hospital were studied to determine the prevalence of Parkinsonian side effects. A Parkinsonism scale was devised and administered. Sixty‐one per cent of the sample had mild to moderately severe side effects. Sex, age, cumulative and current chlorproma‐zine doses, cumulative and current anticholinergic doses and anti‐epileptic medication status did not predict the Parkinsonism scores. Overt brain damage was not a predictor. The difference between the neuroleptic medicated group and neuroleptic free matched controls was highly significant indicating that the Parkinsonian type of movement disorder was related to neuroleptic m