OBJECTIVES: The behavioral, adaptive and quality of life characteristics of attenuated mucopolysaccharidosis type II (MPS II) have not been well studied. Understanding changes over time in the attenuated phenotype may assist in helping achieve better outcomes in long-term function. This longitudinal study investigates these outcomes in relation to age, somatic disease burden, and IQ. Specifically, somatic disease burden is a major challenge for these patients, even with treatment with enzyme replacement therapy. METHODS: 15 patients, 10 between ages 6 and < 12 and 5 between ages >/= 12 and 18, were selected who had at least 2 yearly visits. The occurrence of physical signs, the Physical Symptom Score, and IQ in these two groups was studied as well as the longitudinal association of age with standardized measures of quality of life, adaptive function, and behavioral symptoms as rated by parents and the child's self-report. Slopes by age across and within patients were calculated for these measures. RESULTS: All but one child had hearing loss, most had joint contractures and short stature. Somatic disease burden increased with age. IQ, although normal for most, also improved with age in those under 12 years of age. Physical quality of life decreased while psychosocial quality of life increased with age. Although other adaptive skills were in the broad average range, daily living skills were low at baseline relative to normative data and decreased over time. Behavior ratings indicated improvement in attention and hyperactivity over time. No patient had severe psychopathology, but older children reported an increasing sense of inadequacy and low self-esteem on self-report, presumably due to increasing awareness of differences from peers over time. CONCLUSIONS: Attenuated MPS II patients have increasing somatic disease burden and poor physical quality of life as they develop as well as decreasing self-esteem and sense of adequacy. Psychosocial quality of life, adaptive skills, and attention improve. Recognition of and intervention around these issues will be beneficial to MPS II attenuated patients who have the resources to use such assistance to improve their long-term outcomes.