A seven-year-old, female entire Labrador retriever was presented for acute-onset vomiting and lethargy, associated with weakness and generalised tremors. The clinical, radiographic, ultrasonographic and histopathological findings revealed septic peritonitis which occurred secondarily to unilateral pyometra and ovarian bursal abscessation. However, in this case, the initial clinical findings, blood parameters, radiographic and ultrasonographic findings did not allow a specific diagnosis. Repeat monitoring was required, and abdominocentesis proved to be the most useful diagnostic test, allowing a definitive diagnosis and the decision to be made as to whether or not to carry out exploratory surgery.