The difficulty in differentiating progressive supranuclear palsy (PSP, also called Steele-Richardson-Olszewski syndrome) from other related disorders was the incentive for a study to determine the clinical features that best distinguish PSP. Logistic regression and classification and regression tree analysis (CART) were used to analyse data obtained at the first visit from a sample of 83 patients with a clinical history of parkinsonism or dementia confirmed neuropathologically, including PSP (n = 24), corticobasal degeneration (n = 11), Parkinson's disease (PD, n = 11), diffuse Lewy body disease (n = 14). Pick's disease (n = 8) and multiple system atrophy (MSA, n = 15). Supranuclear vertical gaze palsy, moderate or severe postural instability and falls during the first year after onset of symptoms classified the sample with 9% error using logistic regression analysis. The CART identified similar features and was also helpful in identifying particular attributes that separate PSP from each of the other disorders. Unstable gait, absence of tremor-dominant disease and absence of a response to levodopa differentiated PSP from PD. Supranuclear vertical gaze palsy, gait instability and the absence of delusions distinguished PSP from diffuse Lewy body disease. Supranuclear vertical gaze palsy and increased age at symptom-onset distinguished PSP from MSA. Gait abnormality, severe upward gaze palsy, bilateral bradykinesia and absence of alien limb syndorme separated PSP from corticobasal degeneration. Postural instability successfully classified PSP from Pick's disease. The present study may help to minimize the difficulties neurologists experience when attempting to classify these disorders at early stages.