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 (P<0.001) better than in patients with progressive disease, but the level of response had no influence in this respect. A plateau of at least 3 months was achieved in 81%, at least 6 months in 74%, at least 12 months in 59% and at least 24 months in 33%. Groups with significantly different (P<0.001) survivals were identified: a plateau of <3 months, 3-11 months, 12-23 months, and 24 months or longer, with median survivals of 10, 27, 46 and 81 months, respectively. In the multivariate analysis of pretreatment variables, only haemoglobin (Hb) (P<0.001) and creatinine at 2 months (P<0.01) were significant for a plateau >12 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.