Methods for rating the severity of psoriasis in a patient are provided. Such methods comprise assessing the percent of the patient's body surface exhibiting signs of psoriasis, assessing the overall elevation, erythema, and scale of the patient's psoriasis plaques, and classifying the severity of the patient's psoriasis into at least four classes based upon the percent of the patient's body surface exhibiting signs of psoriasis adjusted by the overall elevation, erythema and scale of the patient's psoriasis plaques. In the present methods the weight given to one or more of the overall qualities of elevation, erythema or scale of the patient's psoriasis plaques in classifying the severity of the patient's psoriasis into a higher class is greater than the weight given to other of the overall qualities. A computer readable medium and system for practicing the present methods are also provided.