The protein concentration of alveolar edema fluid in acute respiratory distress syndrome (ARDS) is dynamic. It reflects alveolar flooding during acute injury as well as fluid and protein clearance over time. We hypothesized that among ARDS patients treated with low tidal volume ventilation, higher concentrations of protein in mini-bronchoalveolar lavage (mBAL) samples would predict slower resolution of lung injury and worse clinical outcomes. Total protein and IgM concentrations in Day 0 mBAL samples from 79 subjects enrolled in the aerosolized albuterol (ALTA) ARDS Network Albuterol trial were measured by colorimetric assay and ELISA respectively. Linear regression models were used to test the association of mBAL proteins with clinical outcomes and measures of length of illness, including ventilator free days (VFDs). Median mBAL total protein concentration was 1740 μg/mL (IQR 890-3170). Each 500 μg/mL increase in Day 0 mBAL total protein was associated with an additional 0.8 VFDs (95%CI 0.05- 1.6, p-value=0.038). Median mBAL IgM concentration was 410 ng/mL (IQR 340-500). Each 50 ng/mL increase in mBAL IgM was associated with an additional 1.1 VFDs (95%CI 0.2- 2.1, p-value=0.022). These associations remained significant and were not attenuated in multivariate models adjusted for age, serum protein concentration, and vasopressor use in the 24 hours prior to enrollment. Thus, higher mBAL total protein and IgM concentrations at Day 0 are associated with more VFDs in patients with ARDS and may identify patients with preserved alveolar epithelial mechanisms for net alveolar fluid clearance.