The study design will be a single-center, randomized, placebo-controlled, cross-over trial. After informed consent has been obtained, 18 eligible subjects with pancreatic insufficient cystic fibrosis will be enrolled in the study. Participants will take part in two 4 week study sessions, each separated by a 4 week washout period. One session will involve treatment with placebo and the other two sessions will provide treatment with approximately 25mg and 35 mg of DHA/kg of body weight. The patients will be assigned to each of the treatment sessions in random order, as described above. The DHA source will be provided by Martek Biosciences Corporation, Columbia, MD, USA in the form of a chewable capsule containing 200 mg of DHA. The placebos will be identical to the DHA supplement but will not contain the active ingredient, DHA. Subjects will be instructed to take the study capsules in addition to their normal doses of pancreatic enzymes with meals and to maintain their usual diets. Blood, urine, and exhaled breath condensate samples will be collected at baseline and after completion of each of the study periods. Patients will be screened and enrolled when they present to clinic for their routine check-up. The subjects have routine blood work at their annual check-ups, and when possible will have an additional tube of blood saved for the baseline fatty acid profiles so as to avoid unnecessary blood draws. Following each study period, blood draw, urine collection, and exhaled breath condensate (EBC) will be collected at the Vanderbilt Clinical Research Center. The patients will also be given the supply of DHA and placebo (for the entire study) at time of enrollment. The order in which they take the supplement or the placebo will be determined using a randomization table.