Fish contain methylamines, especially trimethylamine N‐oxide (TMAO), trimethylamine (TMA), and dimethylamine (DMA). Further, DMA may be formed from TMA and TMAO. DMA is a precursor of nitrosodimethylamine (NDMA), which is a potent carcinogen. Levels of DMA, TMA, and TMAO in urine were used as indicators of the dietary exposure and in vivo formation of these amines in 44 men, representing 3 groups with different fish consumption habits. The levels of TMA (median 0.24 mmol/mol creatinine; range 0–2.7) and TMAO (median 38 mmol/mol creatinine; range 8–290) were significantly associated with the weekly intake of fish (r = .47, p = .001, and r = .53, p = .0002, respectively), while no such relation was found for DMA (median 24 mmol/mol creatinine; range 5–46). Further, urinary levels of TMA and TMAO were dependent on recent intake of fish.