In boys with constitutional delay of growth and puberty, adult height may be inconsistent with parental (target) height. We aimed at studying which period of growth was important to account for adult height being above or below target height. In this retrospective study, adult height measured after 20 years in 39 patients was compared with target height and height data obtained at about 6 and 12 years of age and at diagnosis of delayed puberty (mean 14.6 years). Twenty-eight patients were untreated while 11 received testosterone enanthate (50 or 100 mg/month for 6 months). The growth data from both groups were pooled since they were not different. On average, the adult height standard deviation score (–0.6 ± 0.8, mean ± SD) was similar to target height (–0.5 ± 0.6). There were, however, marked individual differences since adult height varied between 1.7 SD (11 cm) below target height and 1.4 SD (9.5 cm) above target height. Multiple regression analysis showed that the most significant determinant of the difference between adult height and target height was height catch up during puberty (p < 0.002). We conclude that the magnitude of height catch up during puberty is a significant determinant of adult height in boys with constitutional delay of growth and puberty. Thus, optimizing pubertal growth may be a relevant therapeutic aim for adult height in boys with short stature and delayed puberty.