Social anxiety disorder (SAD) and avoidant personality disorder (AvPD) are frequently co-occurring psychiatric disorders with symptomatology related to fear of social situations. It is uncertain to what degree the 2 disorders reflect the same genetic and environmental risk factors. The current study addresses the stability and co-occurrence of SAD and AvPD, the factor structure of the diagnostic criteria, and genetic and environmental factors underlying the disorders at 2 time points. SAD and AvPD were assessed in 1,761 young adult female twins at baseline and 1,471 of these approximately 10 years later. Biometric models were fitted to dimensional representations of SAD and AvPD. SAD and AvPD were moderately and approximately equally stable from young to middle adulthood, with increasing co-occurrence driven by environmental factors. At the first wave, approximately 1 in 3 individuals with AvPD had SAD, increasing to 1 in 2 at follow-up. The diagnostic criteria for SAD and AvPD had a two-factor structure with low cross-loadings. The relationship between SAD and AvPD was best accounted for by a model with separate, although highly correlated (r = .76), and highly heritable (.66 and .71) risk factors for each disorder. Their genetic and environmental components correlated .84 and .59, respectively. The finding of partially distinct risk factors indicates qualitative differences in the etiology of SAD and AvPD. Genetic factors represented the strongest time-invariant influences, whereas environmental factors were most important at the specific points in time. (PsycINFO Database Record