Background: Drowning accounts for hundreds of preventable deaths in Canada every year, but the impact of preexisting medical conditions on the likelihood of death from drowning is not known. We aimed to describe the prevalence of pre-existing medical conditions among people who fatally drowned in Canada and evaluate the risk of fatal drowning among people with common pre-existing medical conditions. Methods: We reviewed all Canadian unintentional fatal drownings (20072016) in the Drowning Prevention Research Centre Canada's database. For each fatal drowning we established whether the person had pre-existing medical conditions and whether those conditions contributed to the drowning. We calculated relative risk (RR) of fatal drowning stratified by age and sex for each pre-existing medical condition using data from the Canadian Chronic Disease Surveillance System. Results: During 2007-2016, 4288 people fatally drowned unintentially in Canada, of whom one-third had a pre-existing medical condition. A pre-existing medical condition contributed to drowning in 43.6% (n = 616) of cases. Fatal drowning occurred more frequently in people with ischemic heart disease (RR 2.7, 95% confidence interval [CI] 2.5-3.0) and seizure disorders (RR 6.3, 95% CI 5.4-7.3) but less frequently in people with respiratory disease (RR 0.12, 95% CI 0.10-0.15). Females aged 20-34 years with a seizure disorder had a 23 times greater risk than their age- and sex-matched cohort (RR 23, 95% CI 14-39). In general, fatal drowning occurred more often while people were bathing (RR 5.9, 95% CI 4.8-7.0) or alone (RR 1.99, 95% CI 1.32-2.97) and less often in males (RR 0.92, 95% CI 0.88-0.95) or in those who had used alcohol (RR 0.72, 95% CI 0.65-0.80), among those with pre-existing medical conditions. Interpretation: The risk of fatal drowning is increased in the presence of some preexisting medical conditions. Tailored interventions aimed at preventing drowning based on pre-existing medical conditions and age are needed. Initial prevention strategies should focus on seizure disorders and bathtub drownings.