Hydrogen-promoted metastable pitting of iron and its development into stable pitting were first investigated by the combination of the electrochemical noise method and the scanning reference electrode technique (SRET). Time records of current noise show that hydrogen significantly increases the number of current fluctuations, inferring that hydrogen might increase the initiation of pitting, and hydrogen also increases the magnitude of current fluctuations, suggesting that hydrogen might promote the growth of metastable pits. In addition, the duration of the time period for initiation of metastable pitting on the charged specimen is longer than that on the uncharged specimen, supporting that the active sites which induce pitting on the charged specimen might be much more numerous than on the uncharged specimen. An analysis of the power spectrum density shows that the level of the frequency-independent plateau of the charged specimen is higher than that of the uncharged specimen and the roll-off frequency tends to decrease with measurement time of the charged specimens, indicating that hydrogen increases pitting attack and decreases the repassivation rate of metastable pits on iron. Statistical analysis of pitting initiation events With various chloride ion concentrations in test solution shows that chloride ion increases spitting initiation significantly on the uncharged specimen. By contrast, the effect of chloride ions on the pitting initiation on the charged specimen is obscured by the significant increase in the number of detectable pitting initiations due to the presence of hydrogen in the specimen. Monitoring of pitting on uncharged and charged iron using in situ SRET shows clearly that the appearance of pits on the charged specimen is much earlier than that on the uncharged specimen and pitting sites on the charged specimen are more numerous than on the uncharged specimen. The results were discussed in terms of the effects of hydrogen on the passive film on iron and hydrogen-promoted anodic dissolution of iron.