A prospective renal ultrasound study of 134 newborns (49 prematures weighing less than 2500 g and 85 more mature hahies) was undertaken to investigate factors associated with increased renal cortical echogenicity (RCE}. Increased RCE was seen in 39 (29 per cent) babies. It was significantly related to body weight, age, medical status, blood urea nitrogen , and serum creatinine. In view of the interdependence of these factors however, multivariate analysis was performed, which revealed that age was the most important factor associated with increased echogenicity (r = -0.46}, and the other factors had only small independent effects, increasing the multiple r to 0.56. The authors conclude that an infant's immaturitv is the most import