Guinea pigs (Cavia porcellus) are popular companion animals with reported skin diseases, but most reports are accessed from textbooks or review articles.To document skin diseases and their prevalence in companion guinea pigs in northern California, USA, and to investigate predilections for the most common conditions over a 25 year period.Two hundred and ninety three guinea pigs from the hospital population met inclusion criteria.A retrospective study was performed by searching computerized medical records seen at the Veterinary Medical Teaching Hospital (VMTH), School of Veterinary Medicine, University of California from 1 January 1990 to 31 July 2015 using key words relevant to dermatology.Of the 580 guinea pigs seen at the VMTH, 50% (293) had skin disease. Of the 293 cases, 154 (52%) presented for nondermatological reasons. Guinea pigs with skin disease were significantly older than those without skin disease (P = 0.0002); females with skin disease were more likely to have cystic ovaries (P = 0.0203), although these were not always associated with alopecia. Pododermatitis and infestation with Trixacarus caviae or lice were the most common skin diseases. Ivermectin or selamectin was used for ectoparasite treatment. Abscesses unassociated with pododermatitis were the most common nodules. Benign follicular tumours were the most common neoplasm. Despite the frequent mention of dermatophytosis in the veterinary literature, only two cases of Trichophyton mentagrophytes were diagnosed.Cutaneous conditions in companion guinea pigs in the USA are common. Clinicians should include a dermatological examination when examining these rodents regardless of the reason for presentation.