There is a need for better and earlier markers of clinical renal damage in subjects with Type I diabetes. In this prospective study, exercise albumin excretion rates (AERs) were studied longitudinally for a 4-year period in 187 young subjects with Type I diabetes. For this time period, 54% of subjects continued to have normal overnight and exercise AERs, 11% had continuously elevated exercise and overnight AERs, 11% developed an elevated exercise AER with the overnight AER remaining normal, and 12% had a normal overnight AER throughout the study, with initially elevated exercise levels later decreasing to normal. This improvement in exercise AER was associated with improved glycosylated hemoglobin (HbA1) values for 64% of the subjects (p = 0.0004, paired t test). Five percent of subjects, who initially had only an elevated exercise AER, developed a consistently elevated overnight AER. Seven of these nine subjects showed either worsening (greater than 10%) or no improvement in their HbA1 values from the initial to the final study periods. Five percent of subjects continued to have an elevated exercise and normal overnight AER throughout the study. These results show that the elevated exercise AER represents a definite transitional stage between a normal and an abnormal (greater than 30 micrograms/min) overnight AER. In addition, a "window" exists during which an elevated exercise AER may be reversed by improved glucose control, but if this improvement does not occur, progression to an increased overnight AER is likely to result.