Growth was evaluated in a group of 10 infants and children with familial or idiopathic classic renal tubular acidosis in whom alkali therapy was initiated at ages ranging from 8 days to 9.5 yr and administered at dosage schedules documented to sustain correction of acidosis in at least four prolonged observation periods on the Pediatric Clinical Research Ward. When alkali therapy was begun, six patients (four infants and two children) were stunted (height <2.5 SD below mean). Of the four who were not, two infants were too young (<2 wk of age) to have become stunted, and two children had been documented earlier to be nonacidotic. At the start of alkali therapy, the heights of the patients correlated inversely with the maximal possible duration of prior acidosis. WITH SUSTAINED ALKALI THERAPY: (a) each patient attained and maintained normal stature; (b) the mean height of the 10 patients increased from the 1.4+/-4 to the 37.0+/-33 percentile (of a normal age- and sex-matched population); (c) the mean height reached the 69th percentile in the eight patients whose heights could be analyzed according to parental prediction (Tanner technique); (d) the rate of growth increased two- to threefold, and normal heights were attained within 6 mo of initiating alkali therapy in the stunted infants and within 3 yr in the stunted children; (e) the height attained correlated inversely with the maximal possible duration of acidosis (before alkali therapy) only in those patients in whom alkali therapy was started after 6 mo of age, and not in those treated earlier. The amount of alkali required to sustain correction of acidosis increased substantially during the course of treatment in each patient. The maximal alkali requirement ranged from 4.8 to 14.1 meq/kg per day, and in each patient its amount was determined principally by the magnitude of renal bicarbonate wasting.