The therapeutic equivalence of a fixed combination preparation consisting of peppermint oil and caraway oil (PCC, Enteroplant) and the prokinetic agent cisapride (CIS, CAS 81098-60-4) was investigated in a four-week randomized controlled double-blind study with planned adaptive interim analysis. The study comprised 120 outpatients with functional dyspepsia. The efficacy was evaluated in 118 patients. Of these, 60 patients received the enteric-coated combination preparation (2 x 1 capsule containing 90 mg peppermint oil +50 mg caraway oil per day) and 58 patients received the reference preparation cisapride (3 x 10 mg/day). The mean reduction of the pain score (primary variable) recorded on a visual analog scale (VAS) during the four-week treatment was 4.62 points with the peppermint oil/caraway oil preparation. This score was comparable with the mean reduction under cisapride (4.60 points) (p = 0.021; test for equivalence). Equivalence was also found in the secondary variable "frequency of pain" with a reduction by 4.65 points under PCC and by 4.16 points under cisapride carried out on an exploratory basis (p = 0.0034). Comparable results were attained with both treatments in the Dyspeptic Discomfort Score which included the other dyspeptic symptoms as well as intestinal and extraintestinal autonomic symptoms, in the prognosis as appraised by the physician and in the CGI scales (Clinical Global Impressions). Corresponding results were also found in Helicobacter pylori-positive patients and patients with initially intense epigastric pain in the two treatment groups. The combination preparation consisting of peppermint oil and caraway, oil appears to be comparable with cisapride and provides an effective means for treatment of functional dyspepsia. Both medications were tolerated well (adverse events were reported in 12 patients of the PCC group and in 14 patients of the CIS group).