Objective. Hepatitis B virus (HBV) infection remains a significant public health challenge, particularly for immunocompromised patients. Our aim was to evaluate the serologic immunity in immunocompromised rheumatology and inflammatory bowel disease (IBD) patients, assess factors for serologic nonimmunity, and evaluate their response to 1 HBV booster dose. Methods. Immunocompromised rheumatology and IBD patients with completed HBV screening were identified. A chart review was performed to collect demographics, clinical information, baseline HBV serology results, and serologic response to booster vaccination. Serologic nonimmunity was defined as a negative/ indeterminate hepatitis B surface antibody (anti-HBs) level. Results. Among 580 patients, 71% were nonimmune. The highest portion of nonimmune patients were 11–18 years old (P = 0.004). There was no significant difference between immune and nonimmune patients with regards to diagnosis (P = 0.34), age at diagnosis (P = 0.64), duration of treatment (P = 0.07), or type of medications (P = 0.08). Sixty-two percent of those who received a booster vaccine were rescreened, and most (68%) seroconverted. In those 18 years or older, only half seroconverted. Conclusion. Results of this study support the benefit of HBV screening in immunosuppressed patients. Beginning at age 11 years, most patients lacked serologic immunity to HBV. Seroconversion for most patients 11–18 years occurred after 1 booster vaccine. Thus, for immunocompromised patients without recent HBV serologic data, obtaining the HBV serology beginning at age 11 years might be considered. Those 18 years and older were least likely to seroconvert after 1 booster, indicating that they may benefit from receiving the 3-dose HBV vaccine series.