In a prospective study to determine simplified clinical signs predictive of pneumonia in children between 2 months and 5 years of age, and to test the validity of the signs recommended by the World Health Organization, clinical findings were correlated with X-ray evidence of pneumonia in 854 children, 400 with pneumonia and 454 with upper respiratory infections (no pneumonia). A respiratory rate of > or = 50/min in infants 2-6 months of age, > or = 40/min in children 7-35 months, and > or = 35/min in children > or = 36 months was the best discriminator of radiological evidence of pneumonia. Use of a respiratory rate of > or = 50/min instead of > or = 40/min resulted in a 14%, 19% and 32% loss of sensitivity with little gain in specificity in the age groups 7-11 months, 12-35 months and > or = 36 months, respectively. The age-specific respiratory rate (recommended by WHO) and/or chest indrawing, history of rapid or difficult breathing and/or chest indrawing, and nasal flaring were also sensitive and specific indicators of pneumonia in almost all the age groups studied.