Birds of different sex and age are known to differ in morphology and physiology. We investigated leucocyte profiles and body condition of the Savi's warbler (Locustella luscinioides) in relation to sex, age and moulting status. We caught birds during the post-breeding period at the stopover site in northern Poland. Of the leucocyte profiles, we examined the heterophil/lymphocyte ratio (H/L), and the number of leucocytes, lymphocytes and heterophils per 10 000 red blood cells (RBC). We found a significant effect of the pentad × age interaction on the H/L ratio. In adults, body condition (BC, size-adjusted body mass) was similar in both sexes. Among adults, females had higher numbers of leucocytes and heterophils per 10 000 RBC and a lower relative number of lymphocytes compared to males. It is possible that subtle sex differences in parental care at the end of the breeding period account for these differences. They may be too subtle to affect BC, but are visible at some leucocyte variables. Moreover, BC of adults decreased with date, indicating the presence of migratory birds using our study area as a stopover site. Moulting adults had higher numbers of all leucocytes and lower H/L values than unmoulted individuals, suggesting a lack of trade-off between the immune system and moulting.