Feline heartworm (Dirofilaria immitis) infection is an uncommon but clinically significant disease in Korea. A retrospective review of electronic medical records from a secondary referral animal hospital in Ulsan, Korea, identified 2 antigen-positive (1.5%) cases among 130 stray cats tested between 2019 and 2023, while no infections were detected in 298 client-owned cats. As antigen testing may yield false-negative results in cats with male-only infections, the true prevalence is likely underestimated. This report describes the clinical and echocardiographic findings of 2 infected stray cats. Case 1 involved successful long-term management of heartworm-associated respiratory disease, with the cat remaining healthy for 4 years following diagnosis. Case 2 demonstrated persistent evidence of adult heartworms and sudden death after an asymptomatic period of 1 year. Echocardiography in Case 2 revealed multiple hyperechoic double lines within the pulmonary arteries, consistent with intraluminal adult worms. These cases illustrate the diagnostic challenges and variable clinical outcomes of feline heartworm infection, emphasizing the need for increased awareness in Korea.