Measurements of maximum expiratory flows and lung volumes were made on 6 occasions at weekly intervals in 14 children who were known over several years to have severe chronic asthma. 11 of the 14 had persistent lung hyperinflation and marked reduction in maximum expiratory flow rates although there was considerable variability in individual measurements. The other 3 usually had hyperinflation and reduced expiratory flow rates. 3 times daily measurements at home of peak expiratory flow rate did not contribute further to assessment. The results of the study indicated that one or two measurements of maximum expiratory flow calculated from a maximum expiratory flow volume curve and of lung volumes recorded in a body plethysmograph are of value in identifying the child with severe chronic asthma.