Several guidelines for the management of asthma suggest that actual/best function is a useful outcome measure. This implies accurate assessment of best function, and a standard for the proportion of best function to be achieved. Seventeen clinics observed their practice simultaneously during four periods in 1990. The aims of the study included testing a protocol for the assessment of best function, and validating actual/best function as an outcome measure. The proposed target for actual/best function to indicate satisfactory control was 80%. The protocol for assessment of best function required formal trial of steroids if best function was < 70% predicted; with regular recording of peak expiratory flow (PEF) if < 80% predicted. PEF was recorded in 515 and FEV1 in 680 of 767 subjects, following the usual clinic practice. If the protocol for best function was not satisfied, mean actual/best function was no higher than if it was, except when best PEF was < 70% predicted. This suggests the need for a PEF chart in these latter patients. Best function was greater in females than males, but actual/best function was almost identical. Whilst best function declined with increasing intensity of treatment, actual/best function was almost independent of regimen step, particularly in the centre which most closely adhered to the protocol. These results confirm that actual/best function is a valid outcome measure. Mean actual/best was > 80% except for FEV1 in two centres. It is suggested that the target in chronic management of asthma is raised from 80 to 85% of best, when actual/best PEF is used as a spot check in patients believed to be on optimal therapy.