Background: Mucopolysaccharidosis type III (MPS III or Sanfilippo syndrome) is a lysosomal storage disease resulting in progressive neurocognitive decline during childhood and early demise. Its diagnosis may have a great impact on parents, potentially leading to psychosocial problems such as anxiety, depression, parental distress, and posttraumatic stress. Methods: Twenty-six mothers and 19 fathers of 34 Dutch MPS III patients completed the “Hospital Anxiety and Depression Scale” (HADS), the “Distress Thermometer for Parents” (DT-P), and the “Self-Rating Scale for Posttraumatic Stress Disorders” (SRS-PTSD). Independent-sample T-tests and chi-square tests were used to assess differences between parents of MPS III patients and reference groups regarding anxiety and depression (HADS), distress (DT-P), and posttraumatic stress (SRS-PTSD). Results: Mothers met the criteria for clinically relevant anxiety (50%) and depression (34.6%) more frequently compared to reference mothers (p ¼ 0.001). Fathers more often met the criteria for clinically relevant depression (36.8%) compared to reference fathers (p ¼ 0.022). Clinically relevant distress was highly prevalent in mothers (84.6%) and fathers (68.4%) of MPS III patients compared to reference parents (p < 0.01). Finally, the prevalence of PTSD was strikingly higher in both mothers (26.9%) and fathers (15%) than reported in the general Dutch population (respectively, p < 0.001 and p < 0.05). Conclusions: We report a clinically relevant impact of parenting an MPS III patient on psychosocial functioning, which is demonstrated by high levels of anxiety, depression, distress, and a remarkably high prevalence of PTSD. Structural monitoring of the psychosocial functioning of MPS III parents is therefore essential and may be beneficial for the whole family.