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Wine grapes contain a great number of phenolic compounds, especially flavanols, which are related to organoleptic and health promoting properties. In this study, flavanol content and the relationship with the antioxidant capacity of seed extracts from a Grenache × Tempranillo progeny segregating for color and sex were evaluated. (+)-Catechin, (-)-epicatechin, (-)-epigallocatechin, and adducts of two cis-units were detected in all samples. The progeny presented high flavanol composition variability. Genotypes were clustered into 5 different groups based on monomer content and length of proanthocyanidins. Female genotypes contained more monomers and shorter proanthocyanidins with less galloylated subunits than hermaphrodites. In red grapes, the epigallocatechin content in polymers and the main degree of polymerization were higher than in white grapes. Environmental conditions influenced total flavanol content; there was a greater number of proanthocyanidins in warm and dry weather. Higher antioxidant capacity was observed in extracts with higher monomer content, and shorter, and less galloylated proanthocyanidins. Among monomers, cis-forms presented stronger influence on antioxidant capacity. Knowing the genetic factors and environmental pressures that determine the synthesis of these compounds could help design agricultural practices aimed at obtaining products enriched with healthy compounds., Financial support by Instituto de Estudios Riojanos, Gobierno de La Rioja, Spain, is gratefully acknowledged. C. Pesquera-Alegría was supported by a FPI-Universidad de La Rioja predoctoral fellowship and a RED VITIS 2.0 fellowship for a short stay at the Department of Technology, Universitat de Lleida.