Background: Therapeutic effect of allogenic bone marrow transplantation (BMT) on I-cell disease has not been determined. Objective: To evaluate effects of BMT in a patient with I-cell disease. Case report: This girl, diagnosed with I-cell disease at age 8 months, underwent BMT at 14 months of age. She had achieved psychomotor development, though delayed, until 11 years of age, but her psychomotor function began to deteriorate after that time and she died at the age 21 years. Her affected elder sister achieved no psychomotor development at all and died at the age 2 years and 11 months. They both carried p. Q104X/p.R1189X mutations of the GNPTAB gene. Results: Activity of GlcNAc-1-phosphotransferase, alfa-mannosidase and beta-hexosaminidase in peripheral lymphocytes increased to a maximum value of 2.28-, 4.8-and 2.8-folds, respectively, after BMT. Chimerism analysis revealed that 94.8% of genome in the peripheral blood was of donor origin, as assed at 15 years of age. Conclusion/Discussion: BMT apparently restored enzymatic activity in the observed periods. Achievement of psychomotor development until the age 11 years may be attributable to possible therapeutic effect of BMT, as compared with the outcome of the affected sister. The reason of the deterioration remains unclear, as rejection was excluded. Conflict of Interest declared.