The present study communicates the interim experience with the diagnostic value of the canine corticosteroid‐induced alkaline phosphatase isoenzyme (CIAP), qualitatively assessed by agarosegel electrophoresis, for the diagnosis of Hypercorticism (prolonged exposure to increased concentrations of exogenous or endogenous corticosteroids) and Diabetes mellitus. A total of 106 canine serum samples with a total alkaline phosphatase activity greater than 2.90 μkat/l were included. CIAP was identified in 28 out of 29 cases of Hypercorticism, in 7 out of 9 cases of Diabetes mellitus, and in 33 out of 68 cases of various other diseases. Sensitivity and specificity with respect to Hypercorticism were 0.97 and 0.48, respectively, while for Diabetes mellitus sensitivity and specificity were 0.78 and 0.37. From this, the positive and the negative predictive values were calculated using standard formulas. The main conclusion may be summarized as follows: In the dog, only the negative finding of CIAP seems to be of diagnostic value in ruling out Hypercorticism as a cause for an increased total alkaline phosphatase activity.