The plasma lipid alterations characteristic of essential fatty acid deficiency (EFAD) generaly appear one to two weeks after the initiation of fat free parenteral nutritional (PN), and may be associated with a reduction in prostaglandin (PG) formation. In the present study the relationship between exclusion of fat from the diet and changes in intraocular pressure (IOP) and PGE2 plasma levels were studied in 28 patients. The results indicated that three weeks after the omission of dietary fat a significant reduction in IOP levels occurred which persisted throughout the follow-up period of 7 (SD 1.2) weeks. Plasma PGE2 levels were also significantly reduced in patients on fat free PN for three weeks as compared with levels measured while patients were on a fat containing diet. This clinical observation is not yet understood and might be related to changes exerted by fatty acid deficiency.