The measurements presented here characterize the effectiveness of a directional antenna in indoor radio environments. We made wideband measurements at the ISM bands, which are centered around 910 MHz, 2.44 GHz, and 0.8 GHz. An omnidirectional antenna is used at the transmitting end, and a horn antenna with a 3 dH beamwidth of about 45 degrees is used at the receiving end. The envelope of the received complex impulse response was recorded for each of the eight directions the receiver antenna was turned to. The environments we examined are: a partitioned office area, an open-layout office area, an auditorium, and a library. It is found that the received power decreases with frequency, while the delay spread is relatively independent of frequency. At the frequency of 5.8 GHz, both the received power and a performance indicator (power received/delay spread) are strongly dependent on direction. This effect is observable to a lesser extent at 2.44 GHz, and is a weak effect at 910 MHz.