The occurrence of nucleated red blood cells (NRBC) in the peripheral blood of critically ill human patients is associated with increased mortality. In dogs, the presence of NRBCs in peripheral blood has been used as a sensitive and specific marker of complications and outcome associated with heatstroke. However, no study has investigated their prevalence in critically ill dogs. Thus, the aim of this study was to determine the prevalence of NRBCs in the peripheral blood, and to evaluate their occurrence as a prognostic factor in critically ill dogs. One hundred and one dogs were prospectively included; the presence of NRBCs was studied on a daily basis from the time of admission until day 3 in the intensive care unit (or less if discharged or death occurred earlier). Dogs fulfilled at least two systemic inflammatory response syndrome (SIRS) criteria and suffered from various diseases. Survival was defined as being alive 28 days postdischarge from hospital. In 42 dogs, NRBCs were detected at least once; 59 patients were NRBC negative. Mortality was significantly higher in NRBC-positive than NRBC-negative patients (54.8 v 30.5 per cent) (P=0.014). However, this association was not present when anaemic dogs were excluded from the analysis. Detection of NRBCs in the peripheral blood may be an indicator for regenerative anaemia and may have potential for use as a prognostic tool or in addition to established scoring systems in critically ill dogs.