Creola bodies (CrB) are characteristically present in sputa from patients with asthma. We studied the relationship among the numbers of CrB, the course of asthmatic attacks and the concentration of sputum eosinophil cationic protein (ECP). Twenty-two asthmatic attacks in 17 patients were studied. Sputum collections pulmonary function tests were made before or during treatment in patients who came to the outpatient clinic or emergency room. Smears of 100 microliters of sputum were made on glass slides and CrB were studied over the whole area of glass slide stained with Papanicolaou's stain. CrB scores were determined from the number and the size of CrB on each slide. In hospitalized patients the CrB scores, the concentrations of sputum ECP and the severity scores of asthmatic attacks were highest on the day of admission, decreasing in association with each other and returning to the baseline level in approximately 5 days. There were significant correlations among the CrB score, the concentration of sputum ECP and %FEV1.0 (p less than 0.001). The CrB score on the day of clinical appraisal significantly correlated with the number 6f days of treatment needed for remission. These results are in keeping with the hypothesis that eosinophils cause desquamation of respiratory epithelial cells resulting in prolongation of asthmatic attacks. Observation of CrB seemed to be useful as a marker of duration of asthmatic attacks.