This prospective study investigates whether the effect of pneumatic dilation in patients with achalasia can be predicted on the basis of patient characteristics, type of treatment, or results of postdilation investigations. Over a period of 10 years, 54 consecutive patients with newly diagnosed achalasia were treated with pneumatic dilation and underwent pretreatment and posttreatment manometric, radiographic, and scintigraphic investigations. They were followed up every 2 years until the fall of 1991. Among the factors evaluated in the initial examination, only young age adversely affected outcome (P < 0.05). With the exception of the diameter of the dilating balloon, the treatment characteristics had a low predictive value. Postdilation lower esophageal sphincter pressure was the single most valuable factor for predicting the long-term clinical response (P < 0.0005). However, patients with high sphincter pressures and poor treatment results benefited from repeated dilations by having progressively longer remissions. It is concluded that young patients are poor candidates for pneumatic dilation, that treatment should be aimed at near complete inflation of the dilating bag, and that postdilation sphincter pressure may guide further treatment.