The peroxisome proliferator-activated receptor-γ (PPAR-γ) is a ligand-dependent transcription factor that is important in adipocyte differentiation and glucose homeostasis and which depends on interactions with co-activators, including steroid receptor co-activating factor-1 (SRC-1). Here we present the X-ray crystal structure of the human apo-PPAR-γ ligand-binding domain (LBD), at 2.2 Å resolution; this structure reveals a large binding pocket, which may explain the diversity of ligands for PPAR-γ. We also describe the ternary complex containing the PPAR-γ LBD, the antidiabetic ligand rosiglitazone (BRL49653), and 88 amino acids of human SRC-1 at 2.3 Å resolution. Glutamate and lysine residues that are highly conserved in LBDs of nuclear receptors form a ‘charge clamp’ that contacts backbone atoms of the LXXLL helices of SRC-1. These results, together with the observation that two consecutive LXXLL motifs of SRC-1 make identical contacts with both subunits of a PPAR-γ homodimer, suggest a general mechanism for the assembly of nuclear receptors with co-activators.