It is incompatible with the right to freedom of movement if a citizen received, in the host Member State, treatment less favourable than that which he would enjoy if he had not availed himself of the opportunities offered by the Treaty in relation to freedom of movement.
Under Polish legislation, only health insurance contributions paid to a Polish insurance institution can be deducted from income tax. The ECJ case dealt with whether this is against the freedom of movement for persons, within the EU Treaty.
Such a restriction can be justified, under Community law, only if it is based on objective considerations independent of the nationality of the persons concerned and is proportionate to the legitimate objective of the national provisions.
The Court found in favour of the taxpayer concluded that a Member State cannot treat less favourably the residence and taxation of resident taxpayers who, in reliance on the Community rules governing the coordination of social security schemes, pay contributions to a social insurance scheme of another Member State.
The full ECJ judgment is available http://curia.europa.eu/jurisp/cgi-bin/form.pl?lang=EN&Submit=rechercher&numaff=C-544/07.