Background Low vitamin D concentrations have been associated with advanced heart disease and poorer outcomes in people and dogs. Vitamin D status typically is assessed by serum 25(OH)D concentration. However, cats also produce notable amounts of a C‐3 epimer of 25(OH)D (3‐epi).

Hypothesis/Objectives Determine if vitamin D status, estimated by 25(OH)D3 alone or combined with 3‐epi (summation vitD), is lower in cats with cardiomyopathy (CM) compared to clinically normal (N) cats and if indicators of disease severity are associated with vitamin D status.

Animals Privately owned cats, 44 with CM and 56 N.

Methods Cross‐sectional observational study using clinical and echocardiographic findings, diet history, and serum 25(OH)D3 and 3‐epi measurements.

Results Cat age was negatively related to vitamin D status. Summation vitD was lower in CM cats (median = 47.1 ng/mL) compared to N cats (median = 58.65 ng/mL) both before (P = .03) and after (P = .04) accounting for age. However, 25(OH)D3 became nonsignificant between CM and N cats after age was included. Summation vitD was related positively to survival time and fractional shortening (FS), but negatively to left atrial enlargement (LAE) severity, both before and after accounting for age. For 25(OH)D3 alone, only survival time and FS remained significant after including age.

Conclusions and Clinical Importance We report 25(OH)D3 and 3‐epi concentrations in CM and N cats. Age had an important (negative) relationship to vitamin D status. After accounting for age, summation vitD was lower in CM cats. Vitamin D status was related positively to survival time and FS, but negatively to LAE severity.