The main purpose of the study was to evaluate the quality of body mass measurement of children under 5 years of age treated in 21 health care units in the city of Rio de Janeiro. A total of 38 health care professionals were observed, 292 children were weighed, and 41 scales were calibrated. Almost all of the mechanical scales were positioned improperly. The procedure of weighing the child "undressed" was not performed in 29.9% of the cases on adult beam scales. Almost all of the scales were calibrated (98%) and showed a high correlation coefficient (0.999). There were no important differences between body mass values obtained by the health care professionals and those read by the observer. Thus, nutritional classification using either value showed a perfect correlation (Kappa correlation coefficient = 1). Although some procedures in obtaining body mass values were unsatisfactory, they did not affect the nutritional classification of children in the present study.