One in five cancers in women is diagnosed prior to and during a woman's fertile years. Our study evaluates mortality risks in offspring of mothers with history of cancer. From the Swedish Multi-generation Register and the Cancer Register, we identified all 174,893 children whose mother had been diagnosed with cancer between 1958 and 2001. We categorized offspring into those born before (>1 year before), around (within 1 year before and after diagnosis) and after (>1 year after) their mother's cancer diagnosis and compared their risks of death (standardized mortality ratios, SMRs) and causes of death to the background population. Overall, offspring of mothers diagnosed with cancer had no increased mortality risk (SMR, 1.00; 95% confidence interval [CI], 0.97-1.03). Increased mortality risks were found in offspring of mothers with tobacco-related cancers (head and neck, thoracic and cervical) (SMR, 1.23; 95% CI, 1.13-1.33), in children born around their mother's diagnosis (SMR, 1.66; 95% CI, 1.25-2.13) and in children born after their mother's hematopoietic cancer diagnosis (SMR, 2.07; 95% CI, 1.10-3.35). Compared to the background population, children born around their mother's diagnosis were more likely to die of congenital and perinatal conditions. Overall, offspring of women diagnosed with cancer were not at increased risk of death, except for certain subgroups. Timing of pregnancy in relation to diagnosis and cancer site modifies mortality risks in the offspring.