Low bone mineral density (BMD) as a risk factor for fractures has been a long‐standing concern in phenylketonuria (PKU). It is hypothesised that the disease itself or the dietary treatment might lead to a low BMD. Previous studies show conflicting results of BMD in PKU due to differences in age, techniques to assess BMD and criteria used. To assess the prevalence of low BMD and define possible risk factors in a large number of adult, early treated PKU (ETPKU) patients. European centres were invited for a survey, collecting retrospective data including results of dual‐energy X‐ray absorptiometry (DXA) scans of adult ETPKU patients. BMD of 183 adult ETPKU patients aged 18‐46 (median age 28, all females premenopausal) years was lower than in the general population at most skeletal sites but the frequency of low BMD (Z‐score <−2) was at maximum 5.5%. No risk factors for low BMD in PKU patients could be identified. Low BMD occurs only in a small subset of PKU patients. DXA scans should be considered for well controlled patients from age 35‐40 years and up and on indication in those PKU patients considered to be at increased risk for fractures.