Urine color is determined by the ratio of water to other chemicals. If it's clear, that means it's mostly water, but there can be two causes: more water, or less chemicals. If you drink a whole bunch of water, it's likely that it's the former reason. If you haven't drank a lot of water, there could potentially be something wrong with how your body is processing things (for example, your kidneys could not be absorbing toxins from your blood stream, which eventually make their way into your urine).

If you drank a lot of water before having clear urine, you probably don't have anything to worry about. If you haven't, then you might want to see a doctor.

If you want to read more, check out http://sxxz.blogspot.com/2006/07/why-is-urine-yellow.html