I also think a registry is important. It's a bit tricky, especially because the kind of registry that really makes a difference is mandatory - that brings up all sorts of privacy issues. But if they've done it for other diseases, I don't know why they can't do it for PD.

Regarding Pharmacies - I just checked with Bill Bell, and he said he had pursued the idea through to an organization called the National Drug Code (NDC http://www.fda.gov/cder/ndc/ ), that tracks all perscriptions in the US. There would have to be some statistical work done around mailorder stuff, etc. to arrive at an estimate.

Apparently it is something that can be done - it's just terribly expensive (on Bill's budget, anyway), at least on the order of a million dollars - a drop in the bucket for the feds!