BACKGROUND Plasma clearance of iohexol (Omnipaque) is well suited for determination of glomerular filtration rate (GFR) during pregnancy. OBJECTIVE To settle guidelines for the use of iohexol clearance in diabetic pregnancy. METHODS Prospective comparative. METHODS Iohexol (5 ml) was injected intravenously and plasma concentrations determined by high pressure liquid chromatography after 180-240 min in 44 diabetic pregnant women. RESULTS Iohexol clearance was normal in most cases of nephropathy or albuminuria. Iohexol clearance correlated negatively with blood pressure, S-Urate, and S-Creatinine. When S-Urate was < 232 mumol/l or S-Creatinine < 70 mumol/l, iohexol clearance was always normal (sensitivity = 1.0). When, in combination, S-Urate was > 345 mumol/l, and S-Creatinine > 70 mumol/l, iohexol clearance was always subnormal (sensitivity = specificity = Kappa index = 1.0). CONCLUSIONS In diabetic pregnancy, nephropathy or albuminuria cannot solely be used for selection of candidates for iohexol clearance determination. A S-Urate < 230 mumol/l (3.9 mg/dl) or S-Creatinine < 70 mumol/l (0.92 mg/dl) was reassuring for a normal GFR. We suggest iohexol clearance determinations in diabetic pregnancies when S-Urate is > 350 mumol/l (5.9 mg/dl) and/or S-Creatinine > 70 mumol/L.