The pressure-temperature (PT) diagram of proteins proposed by Hawley concerns the equilibrium between native and denatured forms. However, the importance of protein aggregation is increasingly recognized, and it has been suggested that certain aggregated states represent alternative folds of the polypeptide chain. Here, we present a PT-diagram for myoglobin in which we include the aggregated state and suggest to call it a PT-state diagram, as not all boundaries are true equilibrium transitions. We observe by Fourier transform infrared spectroscopy that increasing temperature causes the protein to aggregate, but that a subsequent further temperature increase results in the dissociation of this aggregate. Moreover, we observe that moderate pressures stabilize myoglobin against thermal denaturation. We hypothesize that this effect originates from the volume changes associated with the aggregation transition.