Recent studies show that immune response to rhGAA is associated with infusion related reactions and limited efficacy of treatment. Our study focus on the immune response to ERT in 4 patients with classical infantile phenotype followed up 1-6 years with ERT. All 4 patients had infusion-rate dependend IREs, temporally for 6-9 months in 3 patients, ongoing in 1 patient. No Ig-E antibody formation. Patient 1-3 had low titer IgG antibody formation in the first year of ERT and immuntolerizied there after. Patient 4 had high titer antibody formation without trend for immuntolerization. Patient 1 had partial response to ERT. Patients 2 and 3 had favourable outcome, as they are able to walk. Patient 4 showed initially some improvements followed by rapid detoriation after onset of IREs. Patient 1 is tested CRIM positve. In patients 2-4 CRIM status is not tested, but the mutations are highly suggestive for CRIM negative. In patients 2 and 3 alternative infusion protocols and preventive premedication were used. Immune response to rhGAA was associated with infusion related events. Poor outcome was observed in a patient with high antibody titer. Patients with immuntolerization had favourable outcome. For future immunmodulation strategies are mandatory to prevent high antibody formation. Background: Chitotriosidase (ChT) activity is 100 to 1000-fold higher than normal in Gaucher disease (GD) patients and falls with enzyme replacement therapy. It is not a useful marker in 6% of the population that is homozygous to a 24 bp duplication in exon 10 of CHIT1. The Hospital das Clínicas da Universidade Federal de Minas Gerais is a reference center in Minas Gerais state-Brazil for the treatment of GD type I. Only recently this center has included ChT in the follow-up. Objective: To establish the ChT activity and genotype for CHIT1 in patients with GD type I in treatment (n=26) in the state of Minas Gerais, Brazil. Methods: ChT activities (Hollak et al, 1994) and genotyping of CHIT1 duplication (Hise et al., 2003) in 26 GD patients. Results: Among 26 patients with GD, 18 were homozygous wild-type (HH=69%), 6 heterozygous (Hh=23%) and 2 homozygotes (hh=8%) for CHIT1 duplication. The ChT activity in patients HH (115-32,577, mean= 6,798 nmol/h/ml), Hh (200-4,417, mean=2,106 nmol/h/ml) was, respectively, around 200 and 62-fold greater than in controls (3-68, mean= 34 nmol/h/ml). The hh produced 1-1,4 nmol/h/ml. Background: In the last years lysosomal labs use Chitotriosidase as diagnostic screening parameter for LSDs. We evaluated the median and interquartile range (IQR) of chitotriosidase in 8 lysosomal storage disorders. Methods: We investigated 101 patients with confirmed diagnosis of Gaucher disease, Sphingomyelinase deficiency, Niemann-Pick disease type C, Fabry disease, GM1-Gangliosidosis, A-Mannosidosis and CESD. In 5% of the patients we found lack of chitotriosidase activity due to a common chito1-gen mutation. Results: It is a retrospective cross-sectional study in children and adults. Median chitotriosidase activity in the control group was 56 nmol/ml/h (IQR 42 nmol/ml/h) reflecting normal values. In Gaucher disease (GD: N=26) 18563 (IQR 13020), in Sphingomyelinase deficiency (SMD: N=9) 1111 (IQR 2009), in Niemann-Pick disease type C (NPC: N=35) 827 (IQR 1204), in GM1-Gangliosidosis (N=6) 776 (IQR 1533), in Fabry disease (N=10) 130 (IQR 151), in A-Mannosidosis (N=13) 171 (IQR 121), in one patient with Farber disease 3279 and in CESD 249 nmol/ml/h. Conclusion: Excluding those patients with lack of chitotriosidase activity 100 percent sensitivity was observed for elevated chitotriosidase in GD, SMD, GM1-Gangliosidosis, Farber disease and CESD. Limited specificity was seen in NPC, Fabry disease and A-Mannosidosis. Extreme high activity>5000 nmol/ml/h was exclusively noted in GD. Background: Monitoring efficacy of ERT in Gaucher type 1 by assaying plasma chitotriosiase activity is complicated by prevalence of its benign deficiency in all populations. Alternately HDL cholesterol measurement is advocated and observed to be increased on ERT.Study Design: We evaluated 102 suspected Gaucher type 1 patients and 165 healthy controls to assess chitotriosidase activity amongst Indians. ERT was initiated in 4 patients and both HDL cholesterol and chitotriosidase were assayed in baseline and follow up samples. Results: Gaucher type 1 was diagnosed in 32 patients based on the β glucocerebrosidase activity. Benign chitotriosidase deficiency was obtained in 21% of Gaucher type 1 and 16% of healthy controls. Chitotriosidase activity in remaining patients was significantly elevated and ranged from 5504-28,340 nmoles/hr/ml in Gaucher type 1 as compared to 8-87 nmoles/hr/ml in healthy controls. Three out of the four patients of ERT showed a significant decrease in chitoriosidase ( 17-24%) and a parallel increase in HDL cholesterol (32-42%) as compared to the pre treatment values. Conclusion: Chitotriosidase deficiency is consistent in India. Chitotriosidase reduces remarkably on ERT. Larger studies are required to assess utility of HDL cholesterol as a marker for monitoring the therapy.