To determine the etiology of acute conjunctivitis in children seen in pediatric practice, 99 patients with conjunctivitis and 102 age-and season-matched controls were cultured for aerobic bacteria including Haemophilus influenzae, and for viruses, Chlamydia trachomatis, and mycoplasmas. Agents statistically associated with conjunctivitis included H. influenzae (42% vs 0%), Streptococcus pneumoniae (12% vs 3%), and adenoviruses (20% vs 0%). One of these three etiologic agents was isolated from 71 (72%) of the patients. Simultaneous infection with two pathogens was uncommon. Staphylococcus aureus was equally prevalent in diseased and control eyes; one strain of C. trachomatis was isolated from a control eye. Although there were variations in the clinical features of viral and bacterial conjunctivitis, differentiation in an individual patient was difficult. An adenovirus was isolated from 11 (65%) of 17 patients who had pharyngitis in addition to conjunctivitis. H. influenzae was isolated from 14 (74%) of 19 children who had both otitis and conjunctivitis. Adenovirus conjunctivitis was common in the fall and H. influenzae in winter.