Background: Effective self-care is regarded as essential to the management of heart failure (HF). The influence of self-care on HF decompensation, however, is not well understood. Accordingly, we examined the relationship between self-care and fluid accumulation accompanying worsening HF as indexed by decreasing intrathoracic impedance (Z). Methods: Z data were collected from 58 HF patients with OptiVol enabled devices (Medtronic Inc, Minneapolis, Minnesota). Heart failure self-care was measured with the European Heart Failure Self-care Behaviour Scale. Regression modeling was used to describe the influence of HF self-care on the likelihood of a fluid index (FI) threshold crossing, the number of threshold crossings, and number of days spent above threshold. Results: Patients were elderly (74.98 [SD, 8.12] years), with a mean left ventricular ejection fraction of 26.21% (SD, 9.77%), and 63.7% had class New York Heart Association III HF. Patients were followed up for 317 (SD, 96) days; 65.5% had FI threshold crossings (mean 1.45 [SD, 1.56] crossings), spending an average of 33.8 (SD, 42.4) days above FI threshold. Controlling for age, sex, left ventricular ejection fraction, functional class, and duration of follow-up, each additional point on the European Heart Failure Self-care Behaviour Scale was associated with an increase in the odds of having had an FI threshold crossing (adjusted odds ratio, 1.201; 95% confidence interval, 1.013-1.424; P < .05) and more days spent above FI threshold (incidence rate ratio, 1.051; 95% confidence interval, 1.002-1.102; P < .05). Conclusion: Intrathoracic impedance measurements obtained from implantable devices provide important information regarding the influence of self-care on fluid accumulation in patients with HF.