Previous limnological investigations have identified Waldo Lake as one of the most pristine and chemically dilute lakes in the world. This study focusing on the major ion chemistry of Waldo Lake has confirmed this status. Situated at a topographical divide in the central Oregon Cascades, with a very small watershed (watershed area to lake area ratio = 2) and shallow, porous soils, Waldo Lake chemistry is dominated by precipitation input with relatively less influence from watershed processes. Based on median values, conductivity for Waldo Lake is 3.4 μS·cm−1, while the cation and anion sums are 39 and 29 μeq·L−1, respectively. Acid neutralizing capacity (ANC, also total alkalinity) is 17 μeq·L−1(0.85 mg·L−1as CaCO3), indicating that Waldo Lake is very weakly buffered and highly susceptible to acidification from acidic deposition, although there is no evidence that this has occurred. Chloride (14 μeq·L−1) and sulfate (2.1 μeq·L−1) are derived mainly from precipitation (wet and dry deposition), ANC is derived mainly from weathering of watershed base minerals, and base cations (Ca+Mg+Na+K) are derived from both precipitation (two-thirds) and weathering reactions (one-third). Waldo Lake has been classified as ultraoligotrophic based on the extraordinarily low values of primary productivity and nutrient concentrations. Reported nutrient concentrations vary widely due to the extremely low concentrations and insensitivity of methods employed. However, phosphorus has been identified as the limiting nutrient based on algal bioassay methods. Best estimates based on literature data are total P < 3 ug·L−1, about 50% as inorganic P, and total N < 100 to 200 ug·L−1, more than 90% as organic N. There is a net removal of most inorganic N and P from precipitation in the watershed and lake, but watershed and internal lake processes affecting N and P are not well understood. Estimated total phosphorus loading to the lake is 620 kg y−1and estimated total phosphorus loss from the lake is 81 kg y−1, with about 85% of P retained in the sediments. Waste disposal facilities installed within the last 30 years could substantially increase phosphorus loading and thus alter the ecology of the lake, although remediation measures are now under consideration.