The coupled influence of the initial metallurgical state and the heating rate to austenite on the occurrence of heterogeneous grain growth during austenitization of an ASTM A213 - T91 steel has been studied using a high speed, high resolution dilatometer. To that aim, two-step thermal cycles were designed, with a first step of heat treatment at 780 °C -the usual tempering temperature- and a second step of austenitization "in situ" at different heating rates. In the first step, different starting metallurgical conditions were obtained by "tempering" for increasing times up to 6 hours. In the second step, austenitization was performed at 1, 30 and 50 °C/s. A heterogeneous austenite grain size distribution was observed after 30 min. austenite holding time at 1050 °C when austenitization was performed at 30 and 50 °C/s heating rate for the as-received material. For the "tempered" materials, heterogeneous grain growth at 1050 °C was observed up to 3 hours of tempering time when the heating rate to austenite was 30 °C/s; for times longer than the mentioned value, a homogeneous austenite grain size distribution was obtained. When the heating rate to austenite was 50 °C/s only heterogeneous grain growth was observed up to 6 hours of tempering time.