Reductions in the physiological cortical to medullary signal intensity ratio are found in magnetic resonance scans of the kidney in non-diabetic glomerular disease. Whether this abnormality can also characterise patients with Type 1 (insulin-dependent) diabetes mellitus and nephropathy is not known. We measured the cortical to medullary signal intensity ratio in magnetic resonance images of the kidney in 34 patients with Type 1 diabetes (ten with either clinical proteinuria or raised serum creatinine or both, nine with microalbuminuria, seven with normal urinary albumin excretion and long duration of diabetes and eight with Type 1 diabetes of short duration). The cortical to medullary signal intensity ratio showed a trend to cluster at lower values in the normoalbuminuric patients with normal serum creatinine rather than in the nine healthy individuals, independent of Type 1 diabetes duration (1.47 +/- 0.06 and 1.41 +/- 0.13 vs 1.63 +/- 0.16; five groups Scheffé F-test p = 0.05-0.1). Among the Type 1 diabetic patients, significant reductions in the cortical to medullary signal intensity ratio characterised overt nephropathy (1.19 +/- 0.15, p less than 0.05 vs all groups), but not microalbuminuria (1.47 +/- 0.13, p = NS), concomitantly with low glomerular filtration rate and elevated fractional excretion of uric acid, but independent of glycaemic control. The determinants of the renal cortical to medullary signal intensity ratio in Type 1 diabetes are uncertain. Reductions in the cortical to medullary signal intensity ratio may be a late finding in diabetic nephropathy, and parallel the accompanying impairment in kidney haemodynamics.(ABSTRACT TRUNCATED AT 250 WORDS)