Introduction: Deep abdominal muscles play an important role in controlling the lumbosacral and hip regions and maintaining core stability. As they take part in the muscle bracing, their activation and efficiency increase are elements of prevention and therapy, e.g. in spine-related painful conditions. Complementing traditional therapy with the ultrasound muscle assessment applied both in the course of therapy and as an element of evaluation of its findings may improve the effectiveness of physiotherapeutic procedures. Research objective: The objective of the study was to assess the reliability of measuring of the lateral abdominal wall deep muscles by means of an ultrasound machine in accordance with the modified research procedure. Material and methods: The study comprised 90 people, where 20 (13 women and 7 men) were selected in random to be included in statistical analysis. Their results were used as the basis for calculating the intraclass correlation coefficient ICC 3,1. Apart from a range of preliminary tests including taking the body weight, WHR ratio and skinfold measurement, every person subject to the study underwent an assessment of transverse sections of their lateral abdominal wall deep muscles (TVA, IO, EO, and their common measurement). The test was always conducted by one, the same researcher. Each measurement was taken 5 times with vertical placing transducer and conscientiously following the research procedure presented in the article. Results: Descriptive statistics have been prepared. The average age of the subjects was around 23.5 years (SD 3.14). The reliability of measurements obtained in the studied population proved to be very high and amounted to ICC=0.98. Conclusion: It was achieved very high reliability of measurements in a modified test procedure. Repeating the measurements of deep abdominal muscles five times and with vertical placing transducer may still increase the reliability of the research findings. In view of this high reliability, ultrasound appears to be an objective and available method enriching diagnostics and treatment of muscle dysfunctions in rehabilitative practice.