text: what medical condition cause small dark bruises on the upper thighs? i am a 43 year old man, i sometimes get unexplained bruises on my upper thighs near my groin. these bruise are usually about the size of a quater. see a doctor. unexplained bruising i have read somewhere is the sign of kiddney issues  http://health.yahoo.com/ency/healthwise/aa65427/aa65436%20;_ylt=ahv41fg4.w2px2jtpmk0ntbxurcf