We studied the serum aluminum levels of 30 intensive care patients receiving six daily doses of magaldrate (Riopan) or aluminium hydroxide (Trigastril). In both groups we found a significant rise of the serum aluminium concentration (p less than 0.01) following administration of the antacid solutions. Examination on day 9 and 15 the magaldrate group showed significantly (p less than 0.05) lower aluminium levels than the aluminium hydroxide group. An increase up to the critical serum aluminium level of 100 ng/ml occurred in none of the patients that all had normal or slightly impaired renal function. Therefore routine measurements of serum aluminium levels in patients without renal impairment are not considered necessary following antacid therapy. However, we recommend the use of antacids with an aluminium absorption rate as low as possible.