The goal of this study was to prove that adjustable laparoscopic gastric banding (LAP-BAND) is semipermeable and that luminal adjustment with saline leads to spontaneous fluid loss, luminal widening, and effect loss which makes repeated readjustments necessary. In 64 patients stoma adjustment was performed with saline according to the guidelines of the manufacturer (group 1). In 32 patients hyperosmolar contrast material was used for stoma readjustments with the intention to detect a system leakage after spontaneous fluid loss and spontaneous luminal widening was observed (group 2). After spontaneous luminal narrowing had occurred in group 2, all patients from group 2 and all additional patients (n = 148) underwent stoma (re-) adjustment with iso-osmolar contrast material (group 3). Spontaneous fluid changes which led to spontaneous changes of the luminal width were then analyzed for the different filling substances in each group. Fifty-two patients from group 1 presented with effect loss because a spontaneous luminal widening had occurred secondary to a fluid loss of 0.1-0.2 ml/month. All 32 patients from group 2 presented with increasing obstruction and food intolerance because a spontaneous luminal narrowing had occurred secondary to a spontaneous fluid gain of 0.1-0.3 ml/month. In our patients from group 3, where stoma adjustment was performed with iso-osmolar contrast material, no spontaneous fluid changes were observed and luminal width/degree of obstruction did not change. The LAP-BAND is semipermeable. Stoma adjustment should not be performed with saline in order to avoid spontaneous luminal widening and the need for repeated readjustments. Stoma adjustments with hyperosmolar contrast material are clearly contraindicated since osmotic fluid gain leads to increasing obstruction. Stoma adjustments should be performed using iso-osmolar filling media which provide a stable luminal obstruction.