In recent years, there has been renewed interest in the use of subtle heating (hyperthermia) in the temperature range of 42-45Â°C in the cure and control of cancer. This paper studies the effects of blood perfusion rates on the temperature distributions produced by an array of invasive microwave antennas. The antennas are small linear antennas with diameters of less than 1 mm that can be inserted into tumors through hypodermic needles or catheters. The study is based on numerically solving the bioheat equation using the alternating direction implicit method. The numerical study is based on a two-dimensional model and assumes homogeneous tissue properties. The results indicate that if the blood flow rate is equal to or less than that in resting muscle there should not be large differences compared to the case of no blood flow. However, for blood flow rates ten times that in resting muscle, e.g., in brain, kidney, or active muscle, the temperature distributions are significantly altered. The effects of frequency, antenna number and spacing, and tissue properties are also studied.