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Borderline personality disorder (BPD) often co-occurs with other health conditions, but the role of genetics vs. environment is unclear. In this multigenerational cohort study using Swedish national registers, the authors quantified the co-aggregation of BPD with other phenotypes in families and estimated the genetic vs. environmental contributions. In a birth cohort of individuals born 1973-2001 and their twins, siblings, cousins, parents, and aunts/uncles, the exposure was BPD in the proband. Odds ratios were estimated for 19 psychiatric, 18 somatic, and seven behavioral/injury outcome phenotypes within individuals and relative pairs. Structural equation modeling was used to estimate genetic, common environmental, and unique environmental contributions to phenotypic associations and genetic correlations (r). Of the 2.7 million individuals in the birth cohort (mean age at follow-up end = 31.7 years, SD = 9.8, 48.5% female), 24,547 (86% female) were diagnosed with BPD by follow-up end. Relatives of individuals with BPD had increased risk for various psychiatric, somatic, and behavioral phenotypes, except cystic fibrosis. Psychiatric phenotypes showed the strongest phenotypic and genetic correlations, with equal contributions from genetic and unique environmental factors. The pattern varied for somatic phenotypes, which also had weaker correlations. Sleep disorders showed the strongest genetic overlap with BPD (r = 0.74, 95%CI = 0.39-1.08). Behavioral phenotypes related to BPD symptoms showed strong associations and genetic overlap (e.g., self-harm r = 0.80, 95%CI = 0.55-1.04), whereas accident-related phenotypes had weaker associations and varied genetic/environmental contributions. BPD co-aggregates with many phenotypes, with psychiatric conditions showing the strongest genetic overlap. These findings guide further research on BPD comorbidity causes and interventions.