I am Polar obsessed...seriously, my husband calls it my "sippy cup" as I have one with me at ALL times, even on the nightstand at bedtime. We have at least 8 at our house. The only time they leak is if I don't tighten the lid enough. Since yours is already tight, maybe yours is defective? I put mine in the dishwasher all the time too. Contact Polar...maybe they'll send you a replacement.