We present an overview of polar stratospheric cloud (PSC) measurements obtained by POAM III in the 1999/2000 Northern Hemisphere winter. PSCs were observed at POAM latitudes from mid-November to 15 March. PSCs in the early season generally occurred between 17 and 25 km. The central altitude of the PSC observations, roughly 21 km, is unchanged between November and late January. PSCs were not observed between 7 and 27 February. When they reappeared, they formed at distinctly lower altitudes, centered roughly at 16 km. We also present both qualitative and quantitative comparisons with airborne lidar and in situ balloon measurements of PSCs obtained over the Norwegian Sea and Scandinavia over the 25–27 January time period. We find that the large-scale PSC altitude features and morphology are well reproduced in the POAM measurements. Finally, we use PSC occurrence probabilities, analyzed as a function of ambient temperature relative to the NAT saturation point, to infer irreversible denitrification. This denitrification is observed to maximize in late February at levels of at least 75% in the 19–21 km region, with similar values in the 16–18 km region. No denitrification was inferred above 21 km or below 16 km.