Whole blood collected from koalas admitted to the Australian Zoo Wildlife Hospital (AZWH), Beerwah, QLd, Australia, during late 2006–2009 was tested using trypanosome species-specific 18S rDNA PCRs designed to amplify DNA from Trypanosoma irwini, T. gilletti and T. copemani. Clinical records for each koala sampled were reviewed and age, sex, blood packed cell volume (PCV), body condition, signs of illness, blood loss, trauma, chlamydiosis, bone marrow disease, koala AIDS and hospital admission outcome (‘survival’ / ‘non-survival’) were correlated with PCR results. Overall 73·8% (439/595) of the koalas were infected with at least 1 species of trypanosome. Trypanosoma irwini was detected in 423/595 (71·1%), T. gilletti in 128/595 (21·5%) and T. copemani in 26/595 (4·4%) of koalas. Mixed infections were detected in 125/595 (21%) with co-infections of T. irwini and T. gilletti (101/595, 17%) being most common. There was a statistical association between infection with T. gilletti with lower PCV values and body condition scores in koalas with signs of chlamydiosis, bone marrow disease or koala AIDS. No association between T. gilletti infection and any indicator of health was observed in koalas without signs of concurrent disease. This raises the possibility that T. gilletti may be potentiating other disease syndromes affecting koalas.