Shirts shrink in warm or hot water. A hot or warm wash cycle heats and agitates woven fibers. This process relaxes the fibers, returning them to the natural state they were in before being pulled and stretched to make clothing.
Some fabrics are more prone to shrinkage than others. Cotton and linen are more likely to shrink, as the fibers are bonded with a looser weave. Wool fibers are covered with small, raised scales that snag together when introduced to heat, water and agitation. Clothes made with natural-synthetic blends or bought as prewashed or preshrunk are safe from shrinkage in both cold and hot laundry cycles.