Hyperfiltration occurs early in diabetes mellitus and has been implicated in the development of microalbuminuria. Our aim was to re-examine the controversial relationship between glycaemic control and glomerular filtration (GFR) in normoalbuminuric, normotensive, non-obese patients with short duration Type 1 diabetes mellitus (DM). We studied 75 Type 1 DM patients, 35 male, aged 18-42 years, with a duration of diabetes of 4-8 years. GFR was determined by inulin clearance; hyperfiltration was defined as above 145 ml min(-1) 1.73 m(-2) (equivalent to 2 SD above mean for a control population). Analysis was by paired Student's t-testing and linear regression. GFR correlated significantly with HbA1c (r= 0.47, p < 0.0001) and fructosamine (r= 0.24, p = 0.035). Mean HbA1c and fructosamine in the 13 patients with hyperfiltration was significantly higher than in the rest of the group (HbA1c: 9.2% (95% C.I. 7.9-10.4%) vs 7.6 % (7.2-7.9), p= 0.002; fructosamine: 479 micromol l(-1) (450-507) vs 410 micromol l(-1) (388-432), p = 0.009. This significant difference persisted even when the two highest values of HbA1c or fructosamine were removed from analysis. Effective renal plasma flow, assessed by PAH clearance, also correlated in all patients with HbA1c (r=0.31, p=0.039). We conclude that poor glycaemic control directly correlates with hyperfiltration and renal hyperperfusion in early Type 1 DM.