Experimental animals and human post-mortem studies concerning renal effects of tramadol addiction showed histopathological changes in proximal tubules. This study investigates early changes in function and structure of proximal tubules to tramadol addiction. The study included 44 males (G1 = 23 controls, G2 = 21 tramadol addicts). The measured urinary parameters were: urinary alpha-1-microglobulin (U.1-m), urinary leucine aminopeptidase (U.LAP), urinary N-acetyl--D-glucosaminidase (U.NAG). Urinary tramadol (U.Tr) was measured in G2. In G2, levels of U.1-m, U.LAP and U.NAG were increased in comparison with G1. These changes were insignificant. U.Tr did not show any correlation with any parameter in G2. Tramadol addiction may affect proximal tubules. This study is a Preliminary assessment for the subclinical renal effects of tramadol.