PURPOSE: Oral testosterone undecanoate and transdermal testosterone gel are the testosterone formulas widely prescribed as hormonal replacement for tesosterone deficiency syndrome (TDS) in male patients. We evaluated the changes in serum testosterone level and the effects of these medicines. MATERIALS AND METHODS: The medical records of 162 patients who were diagnosed with TDS based on serum testosterone (<2.55 ng/ml) and prescribed testosterone formula were analyzed retrospectively. In the 111 patients of group I, tablets of oral testosterone undecanoate were initially given. If the follow up serum testosterone level had not increased enough, the dosages were increased up to 6 tablets. In the 51 patients of group II, one pack of transdermal testosterone gel (5 gm) daily without dosage adjustment was prescribed. Statistical analysis was done to identify any correlations among age, initial and final aging male's symptoms scale (AMS) score, initial, peak and final testosterone level, and duration of therapy. RESULTS: The mean ages of group I and II were 53.4+/-14.5 and 55.2+/-11.9, respectively (p>0.05). The initial and final testosterone levels of the two groups were not significantly different. However, the peak level during treatment was significantly higher in group II (p<0.05). The maximal increment of testosterone level was also significantly higher in group II. Initially, group II reached its peak testosterone level earlier than group I. The final serum levels were not significantly different after adjustment of dosages in group I. Testosterone replacement significantly decreased the AMS scales in both groups. CONCLUSIONS: Both oral testosterone undecanoate and transdermal testosterone gel improved the serum testosterone level and symptom score for those with TDS. Transdermal testosterone gel may reach the peak serum testosterone level faster than oral testosterone undecanoate. Large prospective studies are required to assess the precise role of testosterone replacement therapy.