HIPAA is the big one, but it doesn't end there. FERPA for college students. And some state laws also. Or just stay away from the freaking US bureaucracy and do it somewhere else. Your idea has merit, but medical records would be the most difficult place to start. I'd start with some other types of records, putting those on the blockchain. Lots of them are public: birth, death, marriage, wills, etc. Try to find a business niche there and then expand it, would be my suggestion.