Despite recent improvements in the composition of the diet, lower mineral bone density and overweight tendencies are incoherently described in patients with phenylketonuria (PKU). The impact of dietary factors and plasma phenylalanine levels on growth, body mass index, body composition, and bone mineral density was investigated in our cohort of patients with hyperphenylalaninemia (HPA) with or without dietary treatment. The anthropometric, metabolic, body mass index and other nutritional indicators and bone mineral density were compared between the group of 96 treated patients with PKU (58 classic PKU (cPKU) and 38 patients with moderate-mild PKU defined as non-classic PKU (non-cPKU)) and the untreated group of 62 patients with benign HPA. Having compared the treated and untreated groups, there were normal outcomes and no statistically significant differences in body mass index, body composition, and bone mineral density. Lower body height standard deviation scores were observed in the treated as compared to the untreated group (p <0.001), but the difference was not significant when analyzing patients older than 18 years; however, cPKU adults were shorter compared to non-cPKU treated adults (p=0.012). Interestingly, the whole-body fat was statistically higher in non-cPKU as compared to cPKU patients. In conclusion, the dietary treatment ensures adequate nutrition without significant consequences in body mass index, body composition, and bone mineral density. A low protein diet may delay the growth in childhood, but the treated patients gain a normal final height. Mild untreated hyperphenylalaninemia characteristic for benign HPA has no negative physiological effect on bone mineral density.