Serum activity of paraoxonase-1 (PON-1) decreases in canine inflammation. This study aimed to evaluate how frequently PON-1 activity is reduced across different disease categories and to assess its potential prognostic significance. PON-1 activity was measured in 482 samples (435 first visit, 47 follow-up) collected during routine clinical activities. The emergency/first-opinion unit (EF) had the highest frequency of low values (18.3%) and lower median PON-1 activity (170.3 U/mL) compared with other units. The proportion of lower values and median PON-1 values were, respectively, significantly higher and lower in acute/severe diseases compared with chronic/mild diseases (13.3 vs. 7.1%; 177.0 vs. 192.9 U/mL) and in hospitalized compared with non-hospitalized dogs (27.7 vs. 4.5%; 150.0 vs. 193.0 U/mL). Low PON-1 activity may predict the need for hospitalization: values < 45.7 U/mL have a likelihood ratio of 14.4. The proportion of lower values and the median Paraoxonase-1 activity did not differ between survivors and non-survivors (18.2 vs. 25.0%; 162.0 vs. 161.3 U/mL). However, PON-1 activity diminished during hospitalization only in non-survivors. PON-1 activity should be measured in routine practice, especially in the EF; hospitalization may be warranted when results are markedly low. Decreases in PON-1 activity during hospitalization may suggest a negative outcome.