We have determined the lever of phosphorylated insulin-like growth factor binding protein-1 (pIGFBP-1)(1) In serum during two catabolic states: diabetes mellitus and trauma. Human sera were Incubated with [I-125]IGF-I for 2 h followed by non-denaturing PAGE. [I-125]IGF-I/IGFBP-1 complexes from serum co-migrated with a pure p(4)IGFBP-1 standard. Complex formation was specifically inhibited by unlabeled IGF-I. The migration of IGF-I/pIGFBP-1 complexes was retarded by IGFBP-1 antibodies, but not by antibodies against IGFBP-2 or IGFBP-3. Sera from three severely traumatized patients had up to 12-fdd more pIGFBP-1 than sera from age-matched controls. The level of pIGFBP-1 was reduced in all three patients upon hospital discharge. Sera from three patients with insulin dependent diabetes mellitus (IDDM) and severe ketoacidosis (DKA) had more pIGFBP-1 than controls. Administration of insulin to DKA patients lowered the level of pIGFBP-1. The present study shows that IGFBP-1 exists as a free, high affinity, phosphorylated form in vivo during two catabolic states.