As pulmonary diffusing capacity is related to mortality risk and prognosis in patients with heart failure (HF), it is measured frequently. As such, it would be essential to know the week-to-week variability (reproducibility) of pulmonary diffusing capacity for carbon monoxide (DLCO) and nitric oxide (DLNO). This variability would let clinicians understand what a clinically measurable change in DLCO and DLNO would be in these patients.On three different days spanning over ten weeks, 40 H F patients underwent testing for DLCO and DLNO. DLCO was determined after a 4 s and 10 s breath-hold maneuver, while DLNO was determined after a 4 s breath-hold maneuver.Forty heart failure patients (66 ± 10 years; BMI = 28.4 ± 4.6 kg∙m-2; 28 males), that were referred to our clinic were able to complete the protocol. DLCO (4 s breath-hold) and DLNO (4 s breath-hold) were 79 ± 19 % and 59 ± 14 % predicted, respectively. Fifty percent of patients (n = 20) were below the lower limit of normal (LLN, below the 5th percentile) for predicted DLCO (4 s), while 78 % of patients (n = 31) were below the LLN for predicted DLNO. All 16 patients that were below the LLN for DLCO were also below the LLN for DLNO. Over a ten week period, the reproducibility of DLNO (4 s) DLCO (4 s) and DLCO (10 s) was 18.9, 8.2, and 5.9 mL min mmHg-1, respectively.The week-to-week fluctuation in DLNO (4 s), as a percentage, is less than DLCO (4 s) in patients with HF. The reproducibility of DLNO in patients with HF is like that of healthy subjects.