Leukocytoclastic vasculitis of small vessels and predominant involvement of the skin are common features of both hypersensitivity vasculitis (HV) and Henoch-Schönlein purpura. In a study comparing 93 patients with HV and 85 patients with Henoch-Schönlein purpura we found major differences with respect to frequencies and type of organ involvement (gastrointestinal tract, kidneys, skin and joints) which were present in both younger and older patients when analyzed separately. To investigate which clinical criteria best differentiate between these 2 vasculitides, 2 methodologies were employed. A rule requiring 3 or more criteria to be present from a list of 6 yielded 87.1% of correctly classified Henoch-Schönlein purpura cases; and 2 or fewer criteria from the same list of 6 correctly classified 74.2% of HV cases. A classification tree was associated with respective values of 83.5 and 84.9%. The results indicate that HV and Henoch-Schönlein purpura are similar but separable clinical syndromes.