Vitamin D deficiency rickets has only rarely been seen in patients with cystic fibrosis (CF), in contrast to other diseases accompanied by fat malabsorption. The lack of rickets in CF has not been explained previously and led us to examine the vitamin D status of selected CF patients with steatorrhea and absent pancreatic enzymes. Accordingly, blood was obtained for analysis from 12 CF patients, 5-43 years of age. Total vitamin D activity was determined in serum samples by the standard bioassay technique which utilizes rachitic rats and gives a range of 25-400 units/dl serum in normal subjects. In addition, total carotenes and vitamin E were measured in lipid extracts of plasma by colorimetric methods. Vitamin E deficiency was found in each of these patients (mean alpha-tocopherol = 92 μg/dl, range = 8-175; normal value = 400 - 1000 μg/dl). All of the samples likewise also showed abnormally low carotene levels (<50 μg/dl). Results of vitamin D bioassays indicated that nine of the CF patients had greater than normal values. Only one showed a deficient level; however, this patient had advanced biliary cirrhosis and was in a terminal condition. Values in the other 11 patients ranged from 236 to 950 units/dl, and the average for the entire group was 604 units/dl. In conclusion, our unexpected finding of high serum vitamin D levels in CF patients, despite their pancreatic achylia, explains the lack of clinical rickets in cystic fibrosis. This may be due to preserved intestinal absorption and/or adequate synthesis of vitamin D in exposed skin surfaces.