Background: Parents of infants suffering from frequent episodes of illness demand more acknowledgement from general practice with regard to their observations of these illnesses, which is evident from their tendency to book multiple consultations. Aim: To identify factors relating to illness and health-care experiences in infancy that predict frequent episodes of illness in toddlers. Design of study: A retrospective questionnaire and a prospective diary study including 183 infants born in February 2001 in a district of the capital region of Denmark. Setting: Denmark, primary care. Methods: Infants were recruited from a birth cohort and experiences of illness from birth until the age of 11 months were collected using a questionnaire. Thereafter the infants were followed prospectively from the age of 11 to 14 months using diary cards. The diary data consisted of 1) selected symptoms, 2) doctor-contacts and 3) parent-rated illness severity, information used to form three aspects of a frequently ill child. The analyses explore associations from the infant data with the three indicators of frequent illness. Results: Experiences of restless sleep, earache, otitis media, penicillin usage and use of medicine associated with illness in infancy were highly associated with factors of excess illness during the follow up period. Disturbed sleep in infancy was the factor with the highest probability of frequent illness as a toddler—an unexpected finding. Experiences of acute otitis media (earache, frequent visits to the doctor and antibiotic treatment) were strongly related to frequent illnesses. Asthma or giving the child medicine correlated with a higher parent-rated frequency of illness later on. Conclusions: Sleep problems in infancy can predict frequent illness episodes later on. In clinical practice the GP may benefit from talking about sleep in the consultations in order to learn something more. The three different indicators of frequent illness applied to different aspects of being a frequently ill child.