Urinary protein/creatinine ratio (Up/cr) is a simple measurement for evaluation of proteinuria. However, exact effects of body size and gender on urinary excretion of creatinine and Up/cr remain unknown. We aimed to clarify their effects. Early morning urine samples were collected from 124 children with urinary tract disorders. Urinary hourly excretion of creatinine, Ucr (in milligrams per hour), urinary hourly excretion of protein per body surface area, Up (milligrams per square meter per hour), and Up/cr (milligrams per milligram) were calculated. Effects of gender, age, body height, body weight and body surface area on Ucr and Up/cr were analyzed, respectively, in a multiple linear regression model. Body surface area and gender affected Ucr (r2=0.842, P<0.0001). Ucr adjusted by body surface area increased as body surface area grew with moderate variation. Up/cr showed a close correlation with Up and was affected by body height and gender as well. The regression equation showed that Up/cr values corresponding to the normal upper limit of Up, i.e., 4 mg/m2/h, in boys and girls 170 cm tall were approximately one third of those in children 80 cm tall (0.121 vs 0.043 for boys, 0.132 vs 0.047 for girls). The present study indicates that estimation of Up/cr needs to include consideration of children's body height and gender.