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In the present study we tried to define the effect of lower respiratory tract infections upon pulmonary function and/or asthma in childhood. Thirty-five children with history of pneumonia in infancy were followed five to ten years later; all were asked to respond questionnaire, received physical examination and were diagnosed for pulmonary function. The results follow: 13 children (37%) had developed asthma, a significantly higher percentage than normal prevalence among students in this area. Simple pulmonary function test, pulmonary function test after distilled water mist and after hypertonic saline (4.5%) mist all showed abnormal values in VC (17%, 14%, 29% respectively), in IVC (46%, 51%, 53%), in FVC (20%, 23%, 24%), in FEVl (17%, 23%, 29%), in FEF25-75% (37%, 49%, 47%), in FEF75% (26%, 23%, 29%) and in FEVl/VC (20%, 14%, 29%). Methacholine challenge test (PC20) showed a marked decrease of PC20 in asthmatic children; each was less than 5 mg/ml (mean value; 0.99 mg/ml). Family-allergy in at least one parent and wheeze were the two significant risk factors. Nevertheless, in 22 non-family-allergy children, the occurrence of asthma was also higher than general prevalence (18.2% vs 5.6%). Wheezing was evident in viral infections in infancy, but bacterial culture from sputum or throat swabs failed to find pathogenic bacteria. These results indicate that while the genetic factor may be important, viral infections may be more important because, even in non-family-allergy children, the occurrence of asthma was higher for infants infected in early infancy than the general prevalence for age-matched students.