Acute respiratory infections (ARIs) are among the leading causes of morbidity and mortality in children under five years in developing countries. Viruses have been long established to be causes of ARIs, but there is little information in developing countries especially Africa. This cross-sectional study was carried out in April-July 2008 in Nairobi, Kenya aiming at establishing which virus isolates and what age cluster in children with ARIs are affected. Throat swabs were collected from patients with clinical ARI and inoculated to various cell lines to isolate and identify viruses. A total of 388 children (age range = 1-132 months) were recruited. Of these 210 (54.1%) were males and 178 (45.9%) were females. Twenty-six percent (101) had upper (URTI) while 74% (287) had lower (LRTI) respiratory tract infections. There were 177 (45.6%) viruses isolated; adenovirus (27; 7%), parainfluenza (25; 6.4%), influenza (20; 5.2%), respiratory syncytial virus (19; 4.9%) and unidentified viruses (86; 22.2%). Majority of the viruses were isolated from patients with LRTI (110; 28.4%) while URTI had 67 (17.3%) isolates. The age cluster 1-12months (105; 27.1%) followed by 13-24months (36; 9.3%) had majority of the viral isolates. Thus this study indicates that viruses are associated with ARIs in children 1-2years of age and therefore a longitudinal study with sensitive virus detection methods is required to provide more information on the precise disease burden and magnitude of the problem. ________________________________________________________________________________