Aims/hypothesis. To evaluate the relationship between metabolic control, kidney function, ambulatory blood pressure and renal morphology in normoalbuminuric adolescents with Type I (insulin-dependent) diabetes mellitus. Methods. Metabolic control, clearance of inulin and para-amino hippuric acid, 24 h ambulatory blood pressure and renal biopsies were studied in 41 patients who were 17.8 ± 0.5 (SEM) years of age and 10.7 ± 0.5 years after onset of diabetes. Results. Glomerular filtration rate and filtration fraction were higher in diabetic patients than in healthy control subjects. At least one third of the patients had systolic and nocturnal diastolic blood pressures above the 90th centile. Basement membrane thickness was 512 ± 17 nm, volume fraction of mesangial matrix 10.7 ± 0.3 % and foot process width 415 ± 6 nm. Nocturnal mean arterial blood pressure, adjusted for body height and gender, correlated directly to the basement membrane thickness, the volume fraction of mesangial matrix and the foot process width. Multiple regression analysis showed that HbA_{1 c}, nocturnal heart rate and body height account for 44 % of the variations in blood pressure. HbA_{1 c}, nocturnal heart rate and body height explained 57 % of the variation in basal membrane thickness, and HbA_{1 c}, nocturnal heart rate and duration of diabetes explained 43 % of Vv(matrix/glom). Actual renal function or urinary albumin excretion rate had no effect. Conclusion/interpretation. Nocturnal heart rate and nocturnal blood pressure, especially the mean arterial blood pressure, seem to be related to glomerulopathy changes in patients in whom persistent microalbuminuria has not yet developed. These findings suggest that a disturbance in sympathovagal balance could have a pathogenic effect. [Diabetologia (2001) 44: 865–873]