This article focuses on the study of the elastic modulus of modern high-performance lightweight concrete. A characteristic feature of concrete, a brittle and composite material, is that its stress-strain characteristics are not exactly linear. This causes some problems in the definition of the Young's modulus. This paper compares a statically determined modulus, called the secant modulus, with a dynamically determined modulus. The obtained results show that the proportions between static and dynamic moduli are significantly different from the ratios known from the literature for normal concretes. For comparison, the results of similar tests for normal concrete with similar strength properties are presented.