The first epidemiological data concerning viruses and asthma were obtained in the 1970s and 1980s by viral isolation and serology. Viral infection can be identified in 24 % to 31.9 % of children, and in 13.3 % of adults. The three most frequent viruses are rhinovirus (RV), respiratory syncytial virus (RSV), and parainfluenza viruses (PIV), detected in 8.8 %, 6.4 % and 6 % of cases, respectively. Due to its amplifying properties, the use of PCR increases the frequency of viral detection, and appears particularly appropriate in asthma where the viral load can be reduced. In a study of bronchiolitis, RSV, PIV3, AdV and RV were identified in 39.3 %, 4.3 %, 1.4 % and 3.9 % of cases, respectively, by IF or culture, and in 62.4 %, 8.3 %, 10.8 % and 12.6 % of cases, respectively, by PCR. Two recent epidemiological surveys used molecular diagnosis in asthma attacks. In a series of 61 adults, 27 (44 %) infections were identified: 16 RV, 4 CV OC43, 3 PIV, 1 RSV, 1 VI, 1 Chlamydia psitacci. In children, viral infection was detected in 226 cases (77 %) : 84 RV, 38 CV, 21 IV, 21 PIV, 12 RSV. We have performed a short retrospective survey for 1997, using molecular biology, on 39 nasal aspirates from children consulting for asthma or wheezing bronchitis. Testing for respiratory viruses by conventional techniques identified 8 (20.5 %) viral infections: 3 RV, 3 RSV, 1 IBV and 1 VPI2. After nucleic acid extraction, PCR-hybridization techniques were applied to these samples to detect RSV, AdV, RV, CV 229E, CV OC43, CP and MP sequences. Twenty six aspirates (54 %) were positive only on molecular biology techniques: 11 RSV, 12 RV, 2 enterovirus, 1 CV OC43. Overall 34 (82 %) viral infections were detected in these children, and a mixed RSV-RV infection was identified in 6 cases. Compared to the studies reported in the literature, we observed the same predominance of RV infections, more RSV infections, probably related to the use of PCR, and a lower incidence of CV infections.