In this retrospective investigation comprising 54 adult patients included in two studies of the treatment of acute asthma, the risk of relapse was studied. The set of variables that best predicted the length of time before a new asthmatic attack comprised blood eosinophil count (P less than 0.001), FEV1 on arrival (P less than 0.001) and increased medication on discharge (P less than 0.01). The set of variables that best predicted the number of new attacks during a period of 1 yr consisted of eosinophil count (P less than 0.001), FEV1 (P less than 0.001), increased medication on discharge, age and breathing rate on arrival (P less than 0.05). Patients with an eosinophil count of more than 500 x 10(6) l-1 and a FEV1 of less tha 1.0 l ran a six-fold greater risk of returning with a new attack within 1 month than patients with lower eosinophil count and higher FEV1. No significant correlation was found between post-treatment PEF variation and relapse in acute asthma.