The therapeutic effect of different diets varying in long chain and medium chain triglycerides, carbohydrate, and protein was tested in two siblings with type I hyperlipoproteinemia. Despite administration of an extremely fat reduced diet ( less than 5 g daily), a normalization of plasma TG could not be obtained because-as a consequence of its high carbohydrate and/or its MCT content-it resulted in a considerable increase in pre-beta-lipoproteins. As life long dietary therapy has to be maintained, the risks of a normal therapy has to be maintained, the risks of a normal fat containing diet (mainly bouts of pancreatitis) and those of a carbohydrate and MCT rich diet (premature atherosclerosis) are to be carefully considered. On the basis of our data we therefore suggest the following dietary regimen: 1. Reduced intake of long chain triglycerides (less than 30 gms per day), but with sufficient amounts of essential fatty acids (4-6 gms linoleate daily). 2. The carbohydrates should not exceed 50% of total calories and ought to consist mainly of starch. 3. The caloric deficit thus generated should be balanced by a high protein intake. This is faciliated by applying a specially protein-enriched food. 4. Medium chanin triglycerides may be necessary when adherence to the protein-rich diet turns out to be bad.