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Most research on dry matter intake (DMI) of lactating dairy cattle has been conducted in the thermal neutral zone, between 5 and 20 degrees C, or under heat stress conditions. Generally studies have found that DMI decreases as the environmental temperature and humidity increases above the thermal neutral zone. Very little information is available on the change in lactating cow DMI in environmental conditions below the thermal neutral zone. The purpose of this study was to measure changes in DMI of lactating cows on three commercial dairy farms in Minnesota and Wisconsin over 12 continuous months. Data collected monthly from an average of 1485 lactating Holstein cows included group average milk production, days in milk (DIM), monthly body weights, DMI and feed refusals. Averaged temperatures and relative humidity levels in the naturally ventilated freestall housing were recorded hourly over the 12-month study period. No significant seasonal effects on DMI of lactating cows were observed on the three farms in this study. Small decreases in DMI during months when the maximum outside temperature exceeded 20 C were observed. No consistent effect on increasing DMI when maximum temperatures remained below the thermal neutral zone temperatures was observed. In the absence of extreme or prolonged humid conditions or temperatures outside the thermal neutral zone, the primary factors affecting DMI of lactating cows on these three farms were milk production and cow characteristics.