['Approximately 40% of adults develop invasive cancer during their lifetimes, many of whom require chemotherapy. Herpes zoster (HZ) is common and often severe in patients undergoing chemotherapy, yet there are no data regarding whether these patients retain specific protection against HZ if they had previously received zoster vaccine. We conducted a study to determine whether zoster vaccine was effective in patients who subsequently underwent chemotherapy.', 'The cohort study consisted of Kaiser Permanente Southern California members aged ≥60 years treated with chemotherapy. The exposure variable was receipt of zoster vaccine prior to initiation of chemotherapy. Incident HZ cases were identified using International Classification of Diseases, Ninth Revision diagnostic codes. HZ incidence rates were calculated; hazard ratios (HRs) and 95% confidence intervals (CIs) were estimated using Cox proportional hazards regression models.', 'There were 91 and 583 HZ cases in the vaccinated and unvaccinated cohorts, respectively, yielding an incidence rate of 12.87 (95% CI, 10.48-15.80) vs 22.05 (95% CI, 20.33-23.92) per 1000 person-years. Thirty-month cumulative incidence was 3.28% in the vaccinated group and 5.34% in the unvaccinated group (P < .05). The adjusted HR for HZ was 0.58 (95% CI, .46-.73) and showed no significant variation by age, sex, or race. HZ incidence rates remained increased in the small subgroup of persons receiving zoster vaccine within 60 days before chemotherapy, but this was the only group affected by indication bias. No vaccinated patients underwent hospitalization for HZ, compared with 6 unvaccinated patients.'] Does vaccination against zoster remain effective in older adults who later undergo chemotherapy?