Atypical antipsychotics (AA) are commonly used to manage drug-induced psychosis (DIP) in parkinsonian patients. In the treatment of schizophrenia, AA’s have been associated with increasing reports of new-onset diabetes mellitus (DM). This study examined the risk of developing new-onset DM among parkinsonian patients on long-term, low dose quetiapine. Fifty-three parkinsonian subjects (mean age: 71.3 years) taking an average quetiapine dose of 70.5 mg/day (range: 12.5–350 mg/day) for a mean duration of 21.3 months (range 3–61 months) were reviewed. Eight out of 53 subjects carried a diagnosis of DM prior to quetiapine treatment. Four out of 45 patients (8.9%) met criteria for new diagnosis of DM, giving a total prevalence rate of 22.6% (12 out of 53). This prevalence rate of 22.6% was slightly higher than that reported in the aged-matched general population (year 2003 DM prevalence = 17.3% for 65–74 years) but methodological differences could explain the difference. Larger epidemiologic studies will be needed to confirm these results as they could potentially impact a significant number of patients.