Background: The increase in the use of monoclonal antibodies (mAB) as a targeted therapy for a variety of diseases has been accompanied by an increase in reports of interstitial lung abnormalities in treated patients. Objective: Bronchoalveolar lavage (BAL) is routinely performed in these patients to rule out infection, so we sought to determine the BAL cellular pattern in individuals with mAB-induced lung disease (mAB-ILD). Methods: We utilized a case-control study design. Among patients treated with mAB, cases were defined as those with otherwise-unexplained interstitial lung abnormalities, which resolved after cessation of treatment, while controls were defined as those with interstitial abnormalities clearly explained by other etiologies. Results: From 2000 to 2012, we identified 9 cases and 7 controls. The mean age of the cases was 62.6 ± 26 years and 6 were female. The most common radiographic finding was diffuse ground-glass opacities. The most common BAL cellular pattern was mixed inflammation with moderate lymphocytic and mild neutrophilic alveolitis. The cases had a higher mean lymphocyte count than the controls (40.1 ± 32.6 vs. 13.1 ± 25.5, p = 0.008). The rest of the BAL cellular analyses were similar between the 2 groups. The median CD4:CD8 ratio in 7 patients with >15% lymphocytes was 0.9 (0.6-3). There was no significant difference in the CD4:CD8 ratio between the 2 groups. Conclusions: Mixed inflammation with moderate lymphocytic and mild neutrophilic alveolitis is the most common BAL cellular pattern in patients with mAB-ILD. Such findings may be useful for the early identification of mAB-ILD.