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Objective To detect temporal case clustering in patients with pityriasis rosea in different geographic locations. Design Regression analysis of dates on which 1379 patients were diagnosed as having pityriasis rosea in 3 different geographic locations. The control data consisted of dates of diagnosis of patients with psoriasis in the same settings. Setting Dermatology clinics in Kuwait, Minnesota, and Diyarbakir, Turkey. Patients Patients with pityriasis rosea and psoriasis seeking care in the clinics. Results Three significant positive clusters (P = .005, P =.001, and P =.01, respectively) and 1 significant negative cluster (P<.001) were detected in these series of patients. No cluster was detected in 2 corresponding series of patients with psoriasis in Kuwait and Turkey. Conclusion Temporal case clustering exists in pityriasis rosea. [ABSTRACT FROM AUTHOR]