Economic growth is affected by the size and dynamics of underground economy. Determining this size is a subject of research for many authors. In this paper we present the relationship between underground economy dynamics and the dynamics of stock markets. The observations are based on regression used by Tanzi (1983) and the relationship between GDP and stock market presented in Tudor (2008). The conclusion of this paper is that the dynamics of underground economy is influenced by dynamic of financial markets. Thus, using specific stock market mathematical tools analysis, one can analyze the dynamic of underground economy