Testosterone and Prostate Cancer
A new paper published in the medical journal Current Treatment Options in Oncology says that "there is a varied and extensive literature indicating that TRT (testosterone replacement therapy) does not pose any increased risk of PCa (Prostate Cancer) growth in men with or without prior treatment."
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Morgentaler A. Testosterone therapy for men at risk for or with history of prostate cancer. Curr Treat Options Oncol. 2006 Sep;7(5):363-9.

From the article abstract:
Since the early 1940s when Huggins showed that severe reductions in serum testosterone by castration or estrogen therapy caused regression of prostate cancer (PCa), it has been assumed that higher testosterone levels cause enhanced growth of PCa.

For this reason, it has been considered taboo to offer testosterone replacement therapy (TRT) to any man with a prior history of PCa, even if all objective evidence suggests he has been cured. The fear has been that higher testosterone levels would "awaken" dormant cells and cause a recurrence. Thus, US Food and Drug Administration-mandated language in all testosterone package inserts states that testosterone is contraindicated in men with a history of, or suspected of having, PCa.

Although there is little modern experience with administration of testosterone in men with known history of PCa, there is a varied and extensive literature indicating that TRT does not pose any increased risk of PCa growth in men with or without prior treatment. For instance, the cancer rate in TRT trials is only approximately 1%, similar to detection rates in screening programs, yet biopsy-detectable PCa is found in one of seven hypogonadal men.

Moreover, PCa is almost never seen in the peak testosterone years of the early 20s, despite autopsy evidence that men in this age group already harbor microfoci of PCa in substantial numbers.

The growing number of PCa survivors who happen to be hypogonadal and request treatment has spurred a change in attitude toward this topic, with increasing numbers of physicians now offering TRT to men who appear cured of their disease. Publications have now reported no prostate-specific antigen (PSA) recurrence with TRT in small numbers of men who had undetectable PSA values after radical prostatectomy.

Although still controversial, there appears to be little reason to withhold TRT from men with favorable outcomes after definitive treatment for PCa. Monitoring with PSA and digital rectal examination at regular intervals is recommended.