OBJECTIVE--To investigate whether during acute asthma episodes a decrease in blood eosinophil count could correlate with the severity of the disease. DESIGN--Prospective study on paediatric asthmatic patients admitted for acute asthma exacerbation between January 1992 and August 1993. All patients were regularly followed up in an outpatient clinic and had had a complete clinical evaluation < 1 month before admission. SETTING--Pulmonary division of the G Gaslini paediatric research institute, Genoa, Italy. SUBJECTS--21 asthmatic patients, 59 (SEM 9) months of age, admitted for acute asthma exacerbation. On the basis of clinical evaluation and the results of blood and microbiological tests performed during acute asthma exacerbations, patients were divided into two subgroups: infected (n = 13) and non-infected (n = 8). RESULTS--All but one of the patients showed a marked decrease in blood eosinophil count during the acute asthma episode, in comparison with recent count (< 1 month before admission) obtained in clinically stable conditions: 662 (116) v 210 (54) eosinophils/mm3, p < 0.0003. The decrease in the eosinophil count was more pronounced in the infected patients than in the non-infected patients, but the difference was not statistically significant (p > 0.05). Similarly, transcutaneous arterial oxygen pressure (PaO2) values measured during acute asthma exacerbations tended to be lower in infected patients, without, however, reaching statistical significance: 8.6 (0.7) v 10.1 (0.9) kPa, p > 0.05). The correlation between the decrease in blood eosinophil count and PaO2 during the acute asthma exacerbations was significant in all the patients (r2 = 0.235, p = 0.022) and in the non-infected patients (r2 = 0.653, p = 0.015), but not in infected patients. In this latter subgroup, a significant negative correlation was found between blood neutrophil counts during acute asthma exacerbations and PaO2 (r2 = 349, p = 0.026). CONCLUSIONS--During acute asthma exacerbations in atopic patients without clinical evidence of infection, the decrease in blood eosinophil count correlates significantly with the decrease in PaO2, further supporting the role of eosinophils in allergic asthma.