The epidermal growth factor (EGF) receptor has been suggested to have an important role in tumor initiation and progression of human bladder cancers. Grb2 protein, which is the downstream effector of the EGF receptor, acts as an adaptor protein between the EGF receptor and the Ras guanine-nucleotide exchange factor, son of sevenless (Sos) protein. Sos protein regulates the action of Ras protein by promoting the exchange of GDP for GTP. However, the significance of Grb2 and Sos proteins, which is related to EGF-triggered Ras activation, has not been elucidated in human bladder cancer. The aim of the present study is to clarify the significance of these proteins in human bladder cancer cell lines. In the present study, we used four human bladder cancer cell lines (T24, KU-7, UMUC-2, UMUC-6) and two kinds of cultured normal urothelial cells (HMKU-1, HMKU-2) isolated from patients with no malignancy. We examined the expression of EGF receptor, Grb2, and Sos proteins in these cells by Western blot analysis. Furthermore, the bladder cancer cell lines were subjected to sequence analysis to identify a point mutation in the c-H-ras gene at codon 12. There was no marked difference in the expression of the EGF receptor between human bladder cancer cell lines and cultured normal urothelial cells. On the other hand, expression of Grb2 and Sos proteins was substantially increased in all human bladder cancer cell lines examined in comparison with cultured normal urothelial cells, whether codon 12 of H-ras was mutated or not. These results suggest that the amplification of both Grb2 and SOS proteins plays an important role in the carcinogenesis of human bladder cancer.