A skin disorder resulting from focal or multifocal dense infiltration of the skin by plasma cell aggregates. It may be associated with high levels of serum IgG4. It typically presents as widespread reddish-brown papules, nodules and pigmented indurated plaques involving the trunk and limbs but may present as a single nodule or plaque. The majority of patients with this uncommon skin disorder are of East Asian descent. There is a risk of progression to systemic lymphoproliferative malignancy.