Importance: California's tobacco control efforts have been associated with a decrease in cancer mortality, but these estimates are based on smoking prevalence of the general population. Patient-level tobacco use information allows for more precise estimates of the proportion of cancer deaths attributable to smoking.

Question: What is the proportion and number of cancer deaths in California attributable to tobacco, using patient-level data?

Findings: In this population-based cohort of 395 459 patients with a tobacco-associated cancer, nearly one-half of cancer deaths were associated with tobacco use, which is almost double what was previously estimated. Current tobacco use was higher than in the general population but decreased over time, leading to a modest but significant decline from 2014 to 2019 in the proportion of cancer deaths associated with tobacco.

Meaning: These findings suggest that tobacco-associated mortality is high but modestly improving in California adults with a tobacco-associated cancer.