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The nutritional and metabolic characteristics of adult phenylketonuria (PKU) patients in the UK with varying dietary adherence is unknown. In other countries, nutritional and metabolic abnormalities have been reported in nonadherent patients compared to adherent counterparts. A pooled analysis of primary baseline data from two UK multi-centre studies was therefore performed to establish whether this is true from a UK perspective. Adult PKU patients who had provided 3-day food records and amino acid blood samples were included and grouped according to dietary adherence (adherent; <i>n</i> = 16 vs. nonadherent; <i>n</i> = 14). Nonadherent patients consumed greater amounts of natural protein compared to adherent patients (61.6 &#177; 30.7 vs. 18.3 &#177; 7.7 g/day; <i>q &lt;</i> 0.001). In contrast, the contribution of protein substitutes to total protein intake was lower in nonadherent compared to adherent patients (3.9 &#177; 9.2 g/day vs. 58.6 &#177; 10.2 g/day; <i>q</i> &lt; 0.001). Intakes of iron, zinc, vitamin D₃, magnesium, calcium, selenium, iodine, vitamin C, vitamin A and copper were significantly lower in nonadherent compared to adherent patients and were below UK Reference Nutrient Intakes. Similarly, intakes of thiamin, riboflavin, niacin, vitamin B₆ and phosphorus were significantly lower in nonadherent compared to adherent patients but met the UK Reference Nutrient Intakes. Phenylalanine concentrations in nonadherent patients were significantly higher than adherent patients (861 &#177; 348 vs. 464 &#177; 196 &#181;mol/L; <i>q</i> = 0.040) and fell outside of European treatment target ranges. This study shows the nutritional and metabolic consequences of deviation from phenylalanine restriction and intake of PKU protein substitutes in nonadherent adult PKU patients. Collectively, these data further underlie the importance of life-long adherence to the PKU diet.