The present paper discusses the predicative instrumental and the alternative grammatical agreement. The first part of the article presents this issue in Polish; the second - introduces a comparative perspective, by competing it with the other Slavic languages: Russian, Ukrainian, Slovakian and Croatian. The article does not discuss the predicative attributes expressed with nouns in the instrumental e.g. 'Poznalam go dzieckiem' (I have known him when he was a child); 'On wraca królem' (He is coming back as a king); 'Przyjechala tu panna' (She has come as a maid here)'. Only attributes expressed with adjectives are regarded as interesting here. As a rule, the attributes concord with a noun and have the nominative or accusative form 'On jest szczesliwy' (He is happy); 'Spotkalem go szczesliwego' (I have met him and he was happy). In theory, they can also have the alternative instrumental form e.g. 'On jest szczesliwym' (He is happy). However this formal difference does not affect the meaning in Polish (as distinguished from Russian and sometimes from Ukrainian). Some verbs connote the instrumental form, and in this case, this form is the only correct one e.g. 'nazwac + Akk + Inst. (call); 'Nazwal mnie glupim / glupia' (He called me stupid). In other cases, the instrumental form is acceptable, along with the nominative or accusative form: 'Spotkalem go chorego II chorym' (I have met him and he was ill). The predicative instrumental is, however, a rather rare phenomenon in Polish and in the other languages analyzed here (with the exception of Russian and partially Ukrainian) and appears only in connection with a small quantity of verbs.