Circulating tumor cells (CTCs) are associated with metastasis of malignant solid tumors in a patient. Presented here is evidence that CTCs exhibit cell cycle phase variability and that there is a strong correlation between the number of CTCs in a mitotic cell cycle phase and the prospects for long term survival of the subject from which the cells were obtained. Also presented herein are methods of determining the mitotic cell cycle phase of CTCs from a patient having cancer and using the information in grading malignant solid tumors and predicting the likelihood of survival of the patient.