Background and aim: To investigate whether veterans involved in chemical warfare agents research at Porton Down have increased rates of mortality or cancer incidence. Methods: The study is a historical cohort study. Participants are male UK veterans who participated in the ‘Service Volunteer Programme’, 1941-1989, identified from Porton Down experiment books, and a comparison group of similar ‘non-Porton Down’ veterans identified from military personnel files. Of 19,233 records retrieved for each group, 18,069 (94%) Porton Down and 17,588 (91%) non-Porton Down are included herein our study sample. The main outcome measures – National Health Service Central Registry Data on mortality and cancer registrations up to December 2019. Results: Over a median follow-up of 48.1 years, 10,889 Porton Down veterans (60.3%) and 10,657 non-Porton Down veterans (60.6%) died. After adjustment for age, year of birth, and military service characteristics, overall, Porton Down veterans had a 7% higher rate of all-cause mortality compared to non-Porton Down veterans. Associations with all-cause mortality were stronger for veterans who attended Porton Down between 1960 and 1964 (1.36, 95% CI 1.20 to 1.54), compared to other periods; likelihood-ratio test, p=0.006. For cause-specific mortality, Porton Down veterans had a statistically significantly higher rates of death from infectious and parasitic disease (5%), genitourinary (45%), circulatory diseases (5%), external causes (23%) and deaths attributable to alcohol (48%). There was no association between attendance at Porton Down and overall cancer incidence (0.99, 0.95 to 1.03). although Porton Down veterans had higher rates of neoplasms of ‘uncertain or unknown behaviour’ (1.26, 1.04 to 1.53), but lower rates of ‘other urinary tract’ neoplasms (0.77, 0.60 to 0.98). Conclusions: Overall, mortality rates were slightly higher in Porton Down veterans, but there was no difference in cancer incidence. Associations were stronger in Porton Down veterans who attended in the early 1960s.