A growing body of research has examined whether birth intervals influence peri-natal outcomes and child health, as well as long-term educational and socioeconomic outcomes; to date, however, very little research has examined whether birth spacing influences long-term health. We use contemporary Swedish population register data to examine the relationship between birth-to-birth intervals and a variety of health outcomes in adulthood, including height, physical fitness, the probability of falling into different body mass index (BMI) categories, and mortality. In models where we do not adjust carefully for family background we find that short and long birth intervals are clearly associated with height, physical fitness, being overweight or obese, and mortality. However, after carefully adjusting for family background using a within-family sibling comparison design, we find that birth spacing is generally not associated with long-term health, though we find that men born after very long birth intervals have a higher probability of being overweight or obese in early adulthood. Overall we conclude that birth intervals have little independent effect on long-term health outcomes.