Bayer posted a fourth quarter net loss of €145 million ($200 million), compared with a profit of €153 million in the same period of 2009, a statement said.
Analysts polled by Dow Jones Newswires had forecast a fourth quarter net profit of €159 million.
Core earnings before interest, taxes, depreciation and amortization (EBITDA) before special items in the quarter gained 11.6 percent on the year to €1.69 billion, a sign the group was still profitable. That result exceeded analyst forecasts for €1.67 billion.
For all of 2010, the maker of Aspirin made a net profit of €1.3 billion, down from €1.36 billion after exceptional charges of €1.72 billion and despite a 12.6 percent gain in sales to a record €35.1 billion.
The charges included a write-down of €405 million to wind down the group's Schering brand and €526 million stemming from US legal costs and settlements following lawsuits targeting Bayer's genetically modified rice.
Operating profit before special items gained 18 percent to €4.45 billion and 2010 EBITDA was 9.7 percent higher at €7.1 billion.
Bayer said earnings at its MaterialScience unit tripled as the global economic rebound boosted demand for construction materials but sales at its HealthCare division were "held back by generic competition and health system reforms."
The group's third main division, CropScience, suffered from a contracting market for conventional crop protection products, Bayer said.
For 2011, it forecast sales growth of between four and six percent to €35-36 billion, assuming the global economy continues to expand, and EBITDA before special items of around €7.5 billion.
Research investment should match the record 2010 level of around €3.1 billion.
"We are confident for this year, which has gotten off to a successful start," chairman Marijn Dekkers said in the statement.