The use of midline catheters has increased to reduce excessive use of central venous access devices, and additional data on midline catheter complications are needed. This study aimed to describe midline catheter complications among hospitalized patients. This retrospective study included a random sample of 300 hospitalized patients with a midline catheter insertion in 2019. The primary outcome was a composite end point of 8 complications: occlusion, bleeding at insertion site, infiltration/extravasation, catheter-related thrombosis, accidental removal, phlebitis, hematoma, and catheter-related infection. Midline catheter failure was defined as removal prior to the end of therapy due to complications. Among 300 midline catheters, the incidence of the composite end point of 1 or more midline complications was 38% (95% confidence interval, 33%–44%). Complications included occlusion (17.0%), bleeding at insertion site (12.0%), infiltration/extravasation (10.0%), catheter-related thrombosis (4.0%), accidental removal (3.0%), phlebitis (0.3%), hematoma (0.3%), and catheter-related infection (0.3%). Midline catheter failure occurred in 16% of midline catheters (n = 48) due to infiltration/extravasation (n = 27), accidental removal (n = 10), catheter-related thrombosis (n = 9), occlusion (n = 4), and catheter-related infection (n = 1). Three catheters had 2 types of failure. The most common complications of occlusion and bleeding rarely resulted in midline catheter failure. The most common causes of midline catheter failure were infiltration/extravasation, accidental removal, and catheter-related thrombosis.