Aims: Acetazolamide enhances decongestion when added to loop-diuretic therapy in patients hospitalized with decompensated heart failure (HF). Whether the timing of acetazolamide initiation is associated with subsequent hospital course remains unclear. We evaluated the association between acetazolamide initiation timing and post-acetazolamide hospital length of stay (LOS).Methods: This single-center retrospective cohort included 77 hospitalized patients with HF who received loop diuretics followed by acetazolamide. The primary exposure was the interval between first loop-diuretic administration and acetazolamide initiation. The primary outcome was post-acetazolamide hospital LOS. We used linear regression with log-transformed LOS for the primary analysis. Restricted cubic splines and sensitivity analyses using alternative modeling approaches were performed.Results: Each 1-day delay in acetazolamide initiation was associated with longer post-acetazolamide LOS in unadjusted (β=0.073, 95% CI 0.041–0.104), demographic-adjusted (β=0.077, 95% CI 0.046–0.108), and clinical-adjusted (β=0.080, 95% CI 0.044–0.115) models (all p<0.001). The clinically adjusted estimate corresponded to an 8.3% longer post-acetazolamide LOS (95% CI 4.5%–12.2%) per additional day of delay. Restricted cubic spline analysis demonstrated an overall association (p<0.001) with evidence of nonlinearity (p=0.009). Findings were consistent across alternative models and analyses restricted to patients discharged alive.Conclusions: Among hospitalized patients with HF receiving loop diuretics followed by acetazolamide, longer time to acetazolamide initiation was associated with longer post-acetazolamide LOS. These findings warrant prospective evaluation of whether treatment timing influences hospital course.