Requirements for a referendum to pass

The decision in the referendum is undertaken if:
1) most positive or negative valid votes is cast in response to the question;
2) most valid votes is cast to one of the variants proposed as solutions to the problem subjected to the referendum.

Art. 235 p. 6 of the Constitution, 

Art. 66 p. 2 of the Law on Nationwide Referendum

Law on Nationwide Referendum adopted on 14 March 2003

http://pkw.gov.pl/g2/i/00/8.pdf (in English - but not updated)

http://isap.sejm.gov.pl/DetailsServlet?id=WDU20030570507 (in Polish – the Parliament’s website)