Year-to-year variations in effective renal plasma flow (ERPF) measurements were determined in 78 patients with spinal cord injury who had no urologic complications or surgery or bouts of chills and fever between consecutive annual follow-up examinations. The effects of age, gender, level and degree of lesion, and time since injury on the degree of variability were also determined. None of the factors considered had a statistically significant effect (P greater than 0.05) on year-to-year variations in ERPF. About 90 per cent of the individual kidney ERPF measurements were within 100 ml/min of the measurement made in the previous year, and 90 per cent of the total ERPF measurements were within 175 ml/min of the previous year's measurement. Follow-up of patients whose year-to-year variability exceeded these limits showed that most subsequently regressed toward the mean spontaneously. These data suggest that kidney blood flow varies considerably from year to year in otherwise healthy patients with spinal cord injury and that changes of 100 ml/min or more for an individual kidney or 175 ml/min or more for both kidneys together are common, but probably of little clinical significance, provided the absolute levels of ERPF remain within normal limits.