Background Recent prevalence studies on hallucinations in idiopathic Parkinson's disease (IPD) reveal that a proportion are of olfactory modality but the clinical features, response to treatment and prognosis of this hallucination modality remain poorly characterised.

Methods 205 consecutively reviewed IPD patients and 205 control patients attending the local hospital were surveyed for the presence of olfactory hallucinations by specific questioning during a consultation and prospectively followed up for at least 3 years.

Results Of 188 patients who had a clinical course remaining consistent with IPD, four were initially found to have olfactory hallucinations, yielding a prevalence of 2.1% (95% CI 0.4 to 5.4%), while two further patients developed such hallucinations later during the study. They were not always accompanied by other hallucination modalities. The patients had a long duration of disease before presentation, loss of sense of smell typical for IPD, a lack of insight into their hallucinations with consequent failure to report them spontaneously, and a good and lasting response to medication adjustment. Unlike patients with visual hallucinations, none developed dementia or features suggestive of non-idiopathic PD over 3 years follow-up. No control patients experienced olfactory hallucinations.

Conclusions The nuisance that olfactory hallucinations cause, their ease of treatment and their failure to be volunteered as a symptom means that specific questioning for their presence should be included in routine assessment of patients with IPD.