Objectives: The aim of this study is to compare the diameter of accessory respiratory muscles in patients with respiratory diseases and patients without such diseases, and to determine whether there is a connection between the ultrasound-measured thickness of accessory respiratory muscles and time of mechanical ventilation, as well as whether there is a connection between the measured thickness and the treatment outcome of patients in the Intensive Care Unit (ICU). Patients and methods: The study was conducted on 50 patients admitted into the ICU. Patients of both genders were included, with and without chronic respiratory diseases, including mechanically ventilated patients. The study was performed using a Mindtray TE7 ultrasound machine with the depth of the linear probe at 70 mm and gain of 90 dB; settings remained unchanged during the study. Measurements of the right m. sternocleidomastoideus, the right m. trapezius, and the right upper leg were taken, with the final value being the average of three measurements for each. The probe was set at the musculoskeletal ultrasound setting. Results: Out of the total of 50 patients, 48 were mechanically ventilated. Patients with chronic pulmonary diseases have a significantly thinner sternocleidomastoideus than patients without chronic pulmonary diseases (Student’s t-test, P=0.007). The older the patient the thinner the femoral quadriceps (r=-0.359, P=0.01). The longer the time of mechanical ventilation the higher the mortality of patients in the ICU (P=0.004). Conclusion: The thickness of the SCM as measured by ultrasound was lower in patients with chronic pulmonary diseases than in patients without such diseases. Correlation was not found between the duration of mechanical ventilation and the thickness of accessory muscles of respiration. The treatment outcome does not depend on the thickness of accessory muscles of respiration.