A 7-month-old Cavalier King Charles Spaniel female was referred due to a chronic cough refractory to antibiotic treatments. Laboratory findings showed leukocytosis, increased serum C-reactive protein, hypogammaglobulinemia, and decreased total serum immunoglobulin G concentration. Thoracic radiographs showed a mild bronchial pattern. Cytology of the bronchoalveolar lavage fluid revealed a septic inflammation. Bordetella bronchiseptica, Mycoplasma spp., and Pneumocystis carinii were identified by polymerase chain reaction testing, and Klebsiella pneumonia was cultured from the bronchoalveolar lavage fluid. Moreover, Escherichia coli was also cultured from urine. Pneumocystis spp. identification was done by sequencing of genetic amplicons. The dog died due to cardiopulmonary arrest secondary to a spontaneous pneumothorax on the day following the procedure. This report documents the detection of Pneumocystis carinii f. sp. canis in a suspected immunocompromised Cavalier King Charles Spaniel with concurrent pulmonary and urinary tract infections involving four different pathogens, and highlights the importance of the use of polymerase chain reaction testing to detect canine Pneumocystis spp. in cases with negative bronchoalveolar lavage cytology.