This report evaluates the June 2008 onychomadesis outbreak in Valencia, Spain. The study sample consisted of 221 onychomadesis cases and 77 nonaffected individuals who lived close to those affected. We collected data on dietary variables, hygiene products, and individual pathological histories. Feces and blood specimens were collected from 44 cases and 24 controls to evaluate exposure to infectious agents. Pathological background data revealed a high frequency (61%) of hand, foot, and mouth disease among the onychomadesis cases. Coxsackievirus A10 was the most commonly detected enterovirus in both case and control groups (49%). Other enteroviruses such as coxsackieviruses A5, A6, A16, B1, and B3; echoviruses 3, 4, and 9; and enterovirus 71 were present in low frequencies in the case and control groups (3-9%). The 2008 onychomadesis outbreak in the metropolitan area of Valencia was associated with an outbreak of hand, foot, and mouth disease primarily caused by coxsackievirus A10.