This study analyzes the long-term effects of delayed motherhood on reproductive fitness and life expectancy of offspring in the mouse. Hybrid (C57BL/6JIco × CBA/JIco) first-generation (F₁) females, either at the age of 10 or 51 wk, were individually housed with a randomly selected 12- to 14-wk-old hybrid male following a breeding pen system until females reached the end of their reproductive life. Reproductive fitness of second-generation (F₂) females was tested from the age of 25 wk until the end of their reproductive life. In F₂ males, the testing period ranged from the age of 52 wk until their natural death. Delayed motherhood of hybrid F₁ female mice was associated with a decreased percentage of male F₃ offspring at birth and lower life expectancy and body weight during adulthood of F₂ offspring. There was, however, no evident negative effect of delayed motherhood on several reproductive fitness variables in either male or female F₂ offspring. This included between-parturition interval, litter size at birth and at weaning, body weight at weaning and preweaning mortality of F₃ pups, percentage of F₃ litters with at least one pup cannibalized, and time at which female and male F₂ offspring ceased their reproductive life. These data clearly show that delayed motherhood in the mouse is associated with negative long-term effects on offspring survival.