OBJECTIVE: To study the relationship between tyrosine phosphorylation (TP) and protein expression of insulin receptor substrate-1 (IRS-1) and insulin resistance in patients with gestational diabetes mellitus (GDM). METHODS: IRS-1 expression and TP in skeleton muscle tissue were determined by Western blot and immunoprecipitation in women with GDM (GDM group, n=22), normal pregnant women (normal pregnancy group, n=22) and normal nonpregnant women (normal nonpregnant group, n=13). Fasting plasma glucose (FPG) and fasting insulin (FINS) were measured by oxidase assay and immunoradioassay. RESULTS: (1) The levels of FPG, FINS, and insulin resistance index were calculated according to homeostasis model assessment [HOMA-IR; (5.6 +/- 0.8) mmol/L, (15.4 +/- 5.1) mU/L, and 1.2 +/- 0.5] in GDM group were significantly higher than those in normal pregnancy group [(4.4 +/- 0.5) mmol/L, (10.6 +/- 3.1) mU/L, and 0.8 +/- 0.3; P<0.01]. The levels of FINS and HOMA-IR in normal pregnancy group were significantly higher than those in normal nonpregnant group [(7.6 +/- 2.3) mU/L and 0.5 +/- 0.3; P<0.01]. (2) The level of IRS-1 protein expression in GDM group (0.64 +/- 0.11) was lower than that in normal pregnancy group (0.81 +/- 0.13; P<0.01). (3) TP with and without insulin stimulation (0.48 +/- 0.14, and 0.35 +/- 0.12) decreased in GDM group, compared with normal pregnancy group (0.66 +/- 0.12, and 0.38 +/- 0.13; P<0.01). TP with insulin stimulation in normal pregnancy group was lower than that in normal nonpregnant group (0.85 +/- 0.09; P<0.01). (4) Protein expression and TP with insulin stimulation of IRS-1 was negatively related to HOMA-IR in GDM group (r=- 0.613, -0.632; P<0.01), and TP with insulin stimulation was negatively related to HOMA-IR in normal pregnancy group (r=-0.526, P<0.05). CONCLUSION: Changes in protein expression and tyrosine phosphorylation of IRS-1 in skeletal muscle may be one of the molecular mechanisms leading to insulin resistance in patients with GDM.