Osteoporosis is a disease of reduced bone density, and it represents a major public health problem that most commonly affects the elderly, more often women than men. In addition to the age-related bone changes, age-related changes in the muscles are also recognized, and they manifest as atrophy and fatty infiltration of the muscles. The gold standard for the osteoporosis diagnosis is a densitometry, method that uses X-rays of two energies, and compares the results with those of a young, healthy population in standard deviations. In recent years, the idea of opportunistic CT screening of reduced bone density has been broadly introduced, showing an excellent correlation with densitometry. In addition, CT has proven to be an excellent method for the diagnosis of muscle infiltration, which is presented as lower values of Hounsfield units (H.U.), confirming that CT can show changes resulting from fat infiltration of only 3%. The aim of this study was to determine the possible association between bone density of the lumbar spine and fatty infiltration of the psoas major muscle on the abdominal CT in young and middle-aged people in whom the obvious degenerative changes are not to expect. We also investigated whether the use of iodine contrast agent affected bone and muscle density measurements in the postcontrast phases of CT imaging. The imaging materials of 113 subjects aged 18 to 49, who underwent a multiphase CT protocol of the abdomen and pelvis from July to December 2021 at the Clinical Hospital Center Split, were retrospectively analyzed. On axial CT slices, H.J. was measured by drawing a circular ROI in the trabecular part of L4 vertebra, and the contours of both psoas muscles were drawn manually and were expressed in anaverage H.U. The results showed a significant age correlation of a decreased bone density and fatty infiltration of psoas muscle with age, and a positive correlation between reduced bone density and fatty infiltration of a muscle. No significant differences were found between the sexes of the subjects, except for higher values of L4 vertebral body density in female subjects on native scans. The application of iodine contrast agent significantly affected the measured values of both parameters. The difference between native and postcontrast measurements of L4 vertebral bone density averaged 20 H.U., and the density of m. psoas major was 6-8 H.U., with postcontrast differences in density of m. psoas being slightly higher in women than in men. The results confirmed the initial hypothesis and we believe that opportunistic CT imaging for other indications can indeed provide valuable data, which can be used to diagnose reduced bone density and fatty muscle infiltration.