There have been allegations for some time now that China systematically harvests and sells the organs of executed prisoners. Thousands of Falun Gong practioneers have been "disappeared" and this may have been their fate.
The BBC has done an undercover investigation that adds a lot of credibility to these rumours;
See the clip on Raw Story. Also check out these telephone transcripts from an earlier independent investigation. In the interests of even-handedness, here's the Chinese Embassy's weasel-worded denial.