The calcium/creatinine ratio (Ca/Creat.) as an indicator of calcium nutritional status was evaluated in "basal urine" from 24 children comprised within the ages of three to 36 months during a 60-day period. The children were divided in normal or undernourished, according to Waterlow's classification. Children under 12 months of age were fed ad libitum with one of two commercial milk formulas, assigned according exclusively to the pediatric criterium. Children older than 12 months received milk besides the habitual hospital diet. In all cases the food intake was controlled and the calcium intake calculated. At the end of the study, basal urine was collected and calcium and creatinine determined. The results revealed that: a) In the normal children the Ca/Creat. ratio gradually decreased with age tending to a low value of 0.2 at the age of 36 months. The relationship between the Ca/Creat. ratio and the calcium intake showed a change in the slope when the requirements were met according to RNC (60 mg/kg/day); b) In undernourished children, the Ca/Creat. ratio was nearly constant whatever their age or calcium intake. The trend to approach the value of 0.2 for calcium intake (about 250 mg/kg/day), however, and the absence of the inflexion point, would suggest that the requirements of these children are far higher than the NRC recommendation, and that the change of the slope would be found for intakes over those obtained in our study. According to these findings, we conclude that the Ca/Creat. ratio in basal urine does not reflect calcium intake, but would depend on the bone calcium turnover. Consequently, it would be a simple and useful indicator of calcium nutritional status.