This paper investigates the indoor thermal conditions in fifty low income non air conditioned houses in Athens, Greece, during the extremely hot summer of 2007. Hourly indoor temperature data have been analysed during three extended heat waves. Indoor temperatures as high as 40 °C were recorded and the average indoor minimum temperature was always above 28 °C. Mean indoor temperatures during the hot season were up to 4.2 K above the normal climatic period, while the daily increase rate varied from 0.2 K/day during the first up to 0.8 K/day for the peak days of the heat wave. The exposure of inhabitants to high temperatures is very important because for almost 85% of the hot period, indoor temperature exceeded 30 °C. Periods of about 216 continuous hours above 30 °C, and six days above 33 °C were recorded in many buildings. The dynamics and the evolution of indoor temperatures as a function of the outdoor climatic characteristics and the thermal capacitance of the buildings are analysed. Possible techniques to improve the indoor environmental quality in low income housing during extremely hot periods are discussed.