Studies of infant teething have been retrospective, small, or conducted on institutionalized infants. To conduct a large, prospective study of healthy infants to determine which symptoms may be attributed to teething and to attempt to predict tooth emergence from an infant's symptoms. Prospective cohort. Setting. Clinic-based pediatric group practice. One hundred twenty-five consecutive well children of consenting Cleveland Clinic employees. Parents daily recorded 2 tympanic temperatures, presence or absence of 18 symptoms, and all tooth eruptions in their infants, from the 4-month well-child visit until the child turned 1 year old. Daily symptom data were available for 19 422 child-days and 475 tooth eruptions. Symptoms were only significantly more frequent in the 4 days before a tooth emergence, the day of the emergence, and 3 days after it, so this 8-day window was defined as the teething period. Increased biting, drooling, gum-rubbing, sucking, irritability, wakefulness, ear-rubbing, facial rash, decreased appetite for solid foods, and mild temperature elevation were all statistically associated with teething. Congestion, sleep disturbance, stool looseness, increased stool number, decreased appetite for liquids, cough, rashes other than facial rashes, fever over 102 degrees F, and vomiting were not significantly associated with tooth emergence. Although many symptoms were associated with teething, no symptom occurred in >35% of teething infants, and no symptom occurred >20% more often in teething than in nonteething infants. No teething child had a fever of 104 degrees F and none had a life-threatening illness. Many mild symptoms previously thought to be associated with teething were found in this study to be temporally associated with teething. However, no symptom cluster could reliably predict the imminent emergence of a tooth. Before caregivers attribute any infants' signs or symptoms of a potentially serious illness to teething, other possible causes must be ruled out.teething, tooth eruption, teeth, deciduous dentition.