With our kitten it was attitude. She picked only a certain family member's clothing to pee on and it turns out that certain family member liked to tease her. Apparently Little Miss Princess decided to get revenge. I started making SURE that clean laundry was put away and if I couldn't get it done then we put something over the laundry basket so she couldn't get in it. We also got after her if we caught her even NEAR the clothes. She stopped.
If you have another cat, your kitten may not feel comfortable sharing the litter box. If you've declawed the kitten recently, he may have sore paws and the litter hurts his toes. If he's 4 - 6 months or older, he may be spraying to mark territory -- get him neutered. A UTI is a possibility.
A hint: wash the clothes in hot water with 1 cup of vinegar. You may have to repeat it but I've found it's very good at getting the urine smell out of clothes, bedding, carpet, etc.
at 11:47 AM on Jul. 29, 2010