OBJECTIVE: To observe the cellular morphology and cytogenetic changes during the treatment of primary and relapsed acute promyelocytic leukemia (APL) with tetrasodium tetrasulphide (TATS) and to explore its mechanism of action. METHODS: Bone marrow short-term culture method, chromosomal fluorescence in situ hybridization (FISH) technique and morphological techniques were applied to analyze cytogenetic and morphological changes in 13 primary and 7 relapsed APL patients. RESULTS: The dynamic changes of bone marrow cell morphology were consecutively observed in 6 primary and 2 relapsed patients at the time of primary or relapse and during TATS treatment, and the results showed that differentiation was observed in the bone marrow APL cells of all 8 patients. Of the total group of 20 cases, 19 showed a gradual decrease in t(15;17)-positive cells during TATS treatment, which correlated with the decrease in the percentage of morphologic APL cells (r-value range 0.7298-0.9989). 19 patients with t(15;17)-chromosomal translocations reached hematological complete remission (CR), 16 reached cytogenetic CR after treatment, and 1 One patient with t(11;17) translocation did not achieve hematological CR. CONCLUSION: TATS has a certain effect on inducing differentiation in primary and relapsed APL. Hematologic and cytogenetic remissions were achieved in patients with primary and relapsed APL treated with single-agent TATS. The application of FISH technology to monitor t(15;17)-positive cells can objectively reflect the changing law of APL cells.