Objective: To estimate the incidence of maculo-papular viral exanthemas and to describe the epidemiological and clinical patterns., Design: Observational descriptive study with a sample design., Participants and Setting: 154 practitioners from the Castilla y León Sentinel Network with a surveilled population of 23 237 people-year under 15 years old, notified in 2002 the cases of diseases by means of a standard form with the variables and inclusion and exclusion criteria., Main Measurements: It was included the maculo-papular exanthemas associated to a presumable systemic virus disease in patients under 15 years old. It was excluded the infectious mononucleose, the chickenpox, and other non viral infections or exanthemas., Results: 368 cases were notified which represent a incidence rate of 158.37 cases per 10 000 (95% CI, 142.31-174.42). The incidence was maximum under four years old, more than 350 per 10 000, decreasing significantly in children over this age. Erythema infectiousum presented the highest rate, followed by exanthema subitum. The exanthemas caused by measles or rubella were insignificants., Conclusions: Childhood exanthematous diseases of presumable viral etiology have an important incidence in primary care, although the majorities are banal and self-limited diseases. Clinical characteristics supported the suspicion diagnosis, which was consistent with the observed epidemiological description and expected presentations of each disease. Although serological analysis could diminish the uncertainly on notification and control of diseases submited to especial programs of vaccination and eradication, they would not improve substantially the diagnosis and treatment of these patients.