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A viral infection often precedes the development of acute otitis media (AOM), and about every fifth URI episode results in AOM.1-3 Most of the symptoms of AOM are caused by the viral respiratory infection, whereas AOM has only few specific symptoms.4, 5 Thus it is often difficult for the parents to know when they should suspect AOM and seek medical help for their child during upper respiratory tract infection (URI). There have been few studies in which temporal development of AOM has been evaluated. In the study of Heikkinen and Ruuskanen6 54% of AOM episodes were diagnosed during the first 4 days and 75% during the first week after the onset of URI. Arola et al.2 found that the mean duration of preceding symptoms before the diagnosis of AOM was 5.9 days.To further evaluate when parents could be advised to contact a physician during a URI, we carried out a prospective study in which the temporal development of AOM was determined. We also analyzed whether children had individual tendencies to develop AOM and whether children with a history of recurrent episodes had a different pattern in developing AOM after the onset of URI.Patients and methods. A total of 857 previously healthy day-care children ages 0.6 to 6.9 years (mean, 3.7 years) were enrolled in a clinical trial for preventing of AOM7 (there were no differences in results concerning temporal development of AOM between intervention group and placebo group). The ethical committee of the Health Center of the City of Oulu approved the study protocol, and informed consent was obtained from the parents. The study was carried out in the Department of Pediatrics, University of Oulu, during a 3-month period (September to December) in 1996. The parents were asked to register daily all the URI symptoms during the study period (fever >38°C, cough, rhinitis, sore throat, vomiting, diarrhea, night restlessness, irritability, poor appetite and conjunctival symptoms). When a child had any of the following acute symptoms, rhinitis, cough, sore throat or conjunctivitis, the child was diagnosed as having URI. The parents were also asked to especially record if their child complained of earache, and in case their child was too young to express it, they were asked to register their own suspicion of earache. The symptoms were recorded daily on a symptom sheet and collected monthly.At the beginning of the study, all the children were screened for MEE with a handhold Micro Tymp (Welch Allyn) minitympanometer and if the finding was abnormal, with pneumatic otoscopy. Children were included into the study only after complete resolution of MEE detected by minitympanometry (A curve) and pneumatic otoscopy (detection of normal motility of tympanic membrane). When a child had any symptoms of URI, the parents were asked to contact the study office and the child was scheduled for an appointment within 3 days. Whenever the child had earache or a sore throat or if the parents suspected AOM, they were asked to bring their child to the office visit on same day. If AOM was not diagnosed at the first visit, the parents were asked to bring their child to the study office immediately when suspecting AOM or weekly until the symptoms resolved. During each visit a trained nurse performed a minitympanometric examination. The tympanogram was repeated at least three times before it was interpreted as abnormal. The tympanogram was classified as abnormal when static admittance (SA) was <0.2 mmho (B-curve) and/or if the tympanic peak pressure (TPP) was <−139 or >+11 daPa. An A curve (SA ≥ 0.