Forceful dilatation under endoscopic control is a well established treatment of achalasia; several different types of dilators can be used. This study prospectively compared the clinical and manometric efficacy of a single dilatation using two different dilators. Forty one patients were randomly assigned to forceful dilatation under endoscopic control with either a pneumatic dilator (n=17) or a metallic dilator (n=24). Thereafter, the patients received periodic clinical and manometric evaluation for one year (before and one, six, and 12 months after dilatation). One month after dilatation all but one of the subjects in each group had experienced good to excellent results and their clinical improvement persisted for the one year follow up. Two patients (one in each group) were perforated during the procedure and required surgical treatment. Recovery was uneventful in both cases. Resting lower oesophageal sphincter pressure (mean (SEM)) significantly and similarly decreased after both methods of dilatation (pneumatic dilator: before dilatation 37 (3) mm Hg, one year after dilatation 18 (3) mm Hg; metallic dilator: before dilatation 34 (2) mm Hg, one year after dilatation 17 (3) mm Hg; p < 0.05 for both). It is concluded that in the treatment of achalasia a single dilatation under endoscopic control with either pneumatic or metallic dilator yield comparable clinical and manometric results and similar complication rates. The use of one or other dilator should depend more on the preference and experience of the endoscopist than on the type of device.