With ion capture cytochemistry, we previously demonstrated the distribution of calcium ions in murine epidermis, a pattern consistent with a role for this ion in the regulation of epidermal differentiation. Because of the known proliferation and differentiation defects in psoriasis, we compared the calcium distribution of involved vs uninvolved psoriatic lesions and normal human epidermis. Whereas normal human and uninvolved psoriatic epidermis revealed increased calcium-containing precipitates in the uppermost stratum granulosum, in contrast the basal layer of psoriatic lesions contained less extracellular calcium, a condition that favored enhanced proliferation. Moreover, all psoriatic suprabasal cell layers displayed heavier than normal concentrations of calcium, indicating loss of the normal calcium gradient that programs terminal differentiation. This abnormal profile may account for the differentiation defects (eg, parakeratosis) that occur in psoriasis. Finally, psoriatic lesions displayed retained ionic Ca in intercellular domains of the upper stratum granulosum with absence of normal intercellular bilayers, findings that may underlie the abnormal desquamation and permeability barrier in psoriasis.