Hyperkalemia is associated with poor prognosis in heart failure (HF), but its reporting in administrative data is likely suboptimal. The aim was to evaluate the frequency and impact of hyperkalemia ICD coding during HF hospitalizations in Brazil. We analyzed 3,551,738 HF hospitalizations from the DATASUS database (2008-2024). ICD codes were used to identify hyperkalemia. Logistic and Cox regression analyses assessed associations between comorbidities, outcomes, and resource utilization. Only 491 hospitalizations (0.014%) were coded for hyperkalemia. These patients were older (mean 70.3 vs. 66.6 years) and had higher rates of chronic kidney disease, diabetes, and cardiovascular disease. Hyperkalemia was associated with increased in-hospital mortality (25% vs. 10%), ICU admissions (17% vs. 11%), dialysis (11% vs. 1%), and longer ICU stays (9.45 vs. 5.36 days). Average costs were 59% higher (2,402 vs. 1,512 BRL). Hyperkalemia is severely underreported in Brazilian HF hospitalizations, despite being a clear marker of clinical severity and higher resource use. Improved awareness and coding may support better outcomes and planning. A hipercalemia está associada a um prognóstico ruim na insuficiência cardíaca (IC), mas seu registro em dados administrativos provavelmente é subótimo. O objetivo foi avaliar a frequência e o impacto da codificação de hipercalemia pela CID durante internações por IC no Brasil. Analisamos 3.551.738 internações por IC do banco de dados DATASUS (2008–2024). Os códigos da CID foram usados para identificar a hipercalemia. Análises de regressão logística e de Cox avaliaram as associações entre comorbidades, desfechos e utilização de recursos. Apenas 491 internações (0,014%) foram codificadas para hipercalemia. Esses pacientes eram mais velhos (média de 70,3 vs. 66,6 anos) e apresentavam maiores taxas de doença renal crônica, diabetes e doença cardiovascular. A hipercalemia foi associada ao aumento da mortalidade hospitalar (25% vs. 10%), internações em unidade de terapia intensive (UTI) (17% vs. 11%), diálise (11% vs. 1%) e maior tempo de internação em UTI (9,45 vs. 5,36 dias). Os custos médios foram 59% maiores (2.402 vs. 1.512 BRL). A hipercalemia é gravemente subnotificada em internações hospitalares por IC no Brasil, apesar de ser um marcador claro de gravidade clínica e maior utilização de recursos. Maior conscientização e melhor codificação podem contribuir para melhores resultados e planejamento. Hyperkalemia is associated with poor prognosis in heart failure (HF), but its reporting in administrative data is likely suboptimal. The aim was to evaluate the frequency and impact of hyperkalemia ICD coding during HF hospitalizations in Brazil. We analyzed 3,551,738 HF hospitalizations from the DATASUS database (2008–2024). ICD codes were used to identify hyperkalemia. Logistic and Cox regression analyses assessed associations between comorbidities, outcomes, and resource utilization. Only 491 hospitalizations (0.014%) were coded for hyperkalemia. These patients were older (mean 70.3 vs. 66.6 years) and had higher rates of chronic kidney disease, diabetes, and cardiovascular disease. Hyperkalemia was associated with increased in-hospital mortality (25% vs. 10%), ICU admissions (17% vs. 11%), dialysis (11% vs. 1%), and longer ICU stays (9.45 vs. 5.36 days). Average costs were 59% higher (2,402 vs. 1,512 BRL). Hyperkalemia is severely underreported in Brazilian HF hospitalizations, despite being a clear marker of clinical severity and higher resource use. Improved awareness and coding may support better outcomes and planning.