The aim of this study was to determine if the production effects of dietary rumen-protected conjugated linoleic acid (RP-CLA) supplemented to dairy cows during the transition period depend on parity. Twenty four Polish Holstein-Friesian pregnant cows and sixteen pregnant heifers were allocated to four treatments: 1. primiparous cows fed a diet without RP-CLA; 2. primiparous cows fed a diet with RP-CLA (60 g/cow/day); 3. multiparous cows fed a diet without RP-CLA; and 4. multiparous cows fed a diet with RP-CLA (60 g/cow/day). The RP-CLA was offered in pre- and postpartum total mixed ration diets from day 21 before expected parturition until day 60 after parturition. Dry matter intake, milk yield and fat, and energy corrected milk yield were not affected by RP-CLA supplementation. RP-CLA supplementation reduced milk fat content by 10.9%, but had no effect on either protein or lactose content, or on yield of fat, protein and lactose. Body condition score (BCS), BCS changes and energy balance (EB) were not affected by RP-CLA addition. Plasma insulin-like growth factor 1 concentration increased in multiparous, but decreased in primiparous cows in response to RP-CLA supplementation (parity × diet interaction). The results of this study confirm that RP-CLA supplementation in transition period lowers milk fat content, but has no effect on milk energy output and EB of the cows. Although multiparous cows had a tendency for a greater decrease in milk fat content due to RP-CLA supplementation, an effect of RP-CLA on milk yield, milk composition and EB in primi- and multiparous cows was similar.