Because glycogen storage disease type IA (GSD-IA) is characterized by recurrent episodes of hypoglycemia that promote a marked elevation in blood triglyceride levels, we evaluated plasma lipid levels in 12 patients with GSD-IA on a regular basis. Six of the 12 patients had plasma fatty acid composition measured; because of possible essential fatty acid deficiency, urinary prostaglandin excretion was also measured. All patients had triglyceride levels between 1440 and 6120 mg/dl (16.25 to 69.09 mmol/L) before treatment. After treatment to promote blood glucose levels of 75 to 85 mg/dl (4.2 to 4.7 mmol/L), triglyceride levels in each of 11 patients were between 189 +/- 31 (2.13 +/- 0.35 mmol/L) and 510 +/- 60 mg/dl (5.76 +/- 0.68 mmol/L). The lipoprotein fatty acid composition in six patients showed a substantial elevation in C16:0, C16:1 omega 7, and C18:1 omega 9, but no increase in C20:3 omega 9 (the fatty acid that characteristically increases in essential fatty acid deficiency). In addition, each of the six patients had normal 24-hour urinary excretion of prostaglandin. One patient, whose triglyceride levels remained elevated despite dietary treatment, was given either clofibrate, lovastatin, niacin, or fish oil. With the exception of lovastatin, these agents produced a decrease in triglyceride values for 1 to 2 months; however, by 3 months triglycerides reached pretreatment levels. Combined treatment with clofibrate and niacin resulted in a sustained decrease in plasma triglyceride levels for 4 months. The findings indicate that dietary management of GSD-IA is usually associated with improvements in triglyceride levels; however, patients maintain triglyceride values between 300 and 500 mg/dl (3.38 to 5.65 mmol/L). No patient had biochemical evidence of essential fatty acid deficiency.