The prognostic value of baseline serum free light chain ratio (sFLCR) was investigated in 94 multiple myeloma (MM) patients. sFLCR was calculated as κ/λ or λ/κ, depending on the patients’ dominating monoclonal light chain. Median baseline sFLCR was 3·57 in κ-MM patients, 45·09 in λ-MM. ‘High’ sFLCR (≥ the observed median value for κ- and λ-MM respectively) correlated with elevated serum creatinine and lactate dehydrogenase, extensive marrow infiltration and light chain type MM. The 5-year disease-specific survival was 82% and 30% in patients with sFLCR lower than and equal or greater than the median, respectively (P = 0·0001). sFLCR was an independent prognostic factor.