Abstract This study was performed to investigate changes in the dendritic cells (DCs) frequency and phenotype in the peripheral blood in Egyptian Type 1 diabetic children. Also to study the level of B, T lymphocytes, activated T lymphocytes, and costimulatory molecules expression on B lymphocytes. Twenty five children with T1DM and 25 healthy controls were enrolled. Flow cytometric detection of DCs, B-lymphocytes and T-lymphocytes, CD19+CD80+, CD19+CD86+, CD19+HLADR+ and CD3+ HLA-DR+ was preformed. The frequencies of monocytoid dendritic cells (mDCs) and plasmacytoid dendritic cells (pDCs) were significantly decreased in diabetic patients than the controls and the mDCs/pDCs ratio was significantly higher in diabetic patients. The expression of costimulatory molecules CD80 and CD86 on the entire DCs was significantly higher in diabetic children. The frequency of pDCs was negatively correlated with the age in diabetic patients and positively correlated with the level of insulin C-peptide. The percentage of CD80 expressing B lymphocytes and of activated T lymphocytes was significantly higher in the patients. Dendritic cells are reduced in number and display more mature phenotype in T1DM children. The higher expression of CD80 on B lymphocytes and activation of T lymphocytes may reflect the ongoing autoimmune process in this disease. Modulation of the DCs could have beneficial effect in T1DM.