Selected water-quality data collected before, during, and after construction of the H-3 Highway at 13 water-quality stations were compared to the State of Hawaii Department of Health water-quality standards to determine the effects of highway construction on the water quality of the affected streams. Highway construction had no effect on the high concentrations of total nitrogen and nitrite plus nitrate nitrogen observed except for increased nitrite plus nitrate nitrogen concentrations at one station on Hooleinaiwa Stream. Exceedences of the 10- and 2-percent-of-the-time concentration standards for total phosphorus, total suspended solids, and turbidity, all constituents associated with sediment, occurred more commonly and at more stations during construction than either before or after. These exceedences may be, in part, due to land disturbance caused by highway construction. Highway construction had no effect on the physical water-quality properties of pH, dissolved oxygen, temperature, and specific conductance except at North Halawa and Kuou Streams, where specific-conductance values increased throughout the study period, most likely due to highway construction. No effects on selected trace metals and organic chemical compounds were observed due to highway construction. No effects due to highway construction were observed in the water quality of Waimaluhia Reservoir. Runoff from areas of urban land use in the Kaneohe drainage basin contributed more to the higher loads of selected water-quality constituents than did runoff from areas affected by highway construction.