DAta from an 8-year follow-up of 432 myeloma patients were analysed for incidence, duration and prognostic value of plateau phase and factors favouring its achievement. The first-line chemotherapy was melphalan and prednisolone in 121 cases, combination chemotherapy in 311 cases. The survival times were similar despite different response rates. Any response resulted in survival significantly (P12 months. After inclusion of chemotherapy data, Hb and time taken to reach the best response were still significant (P=0.002). The predictive power of high Hb and slow response for achieving a plateau of 6 or 12 months was 79%. Accordingly, the criteria for response of treatment for multiple myeloma should include a stable period of at least 3 months; the criteria for a plateau a stable period of at least 12 months in order to have real prognostic significance.