Isolates cases and groups of patients co-infected with human T cell lymphotropic virus type I (HTLV-I) and Norwegian scabies have been previously reported. Peru is considered to be endemic for HTLV-I. Between June 1999 and December 2000, 23 patients with Norwegian scabies were enrolled in this study after written informed consent was obtained. Antibodies against HTLV-I were detected by an enzyme-linked immunosorbent assay and confirmatory Western blot. Patients ranged in age from 3 months to 84 years; 15 (65.2%) were female. Infection with HTLV-I was found in 16 (69.6%) patients. Comorbid features included corticosteroid therapy (8.6%), malnutrition (8.6%), and Down's syndrome (4.3%). Among those who consented to be tested for human immunodeficiency virus (n = 13,56.5%), no one had a positive result. The three patients that reported one or more prior episodes of Norwegian scabies were infected with HTLV-I and two of these HTLV-I-positive patients died. Infection with HTLV-I is an important co-factor related to Norwegian scabies in Peru. In our setting, the evaluation for HTLV-I in all Norwegian scabies cases is highly recommended, especially when no other risk factors are apparent. Copyright © 2005 by The American Society of Tropical Medicine and Hygiene.