evidence: Information on cannabinoids in breast milk and maternal cannabis use is limited. We quantified cannabinoids in plasma and breast milk of breastfeeding mothers and assessed cannabis use patterns. [...] All but a single breast milk sample had at least one cannabinoid or metabolite detected. The sample with no detectable cannabinoids was collected 40 h after last cannabis use. Plasma and breast milk cannabinoid concentrations from both visits combined are summarized in Table 2. There was a moderate positive correlation between plasma THC concentration and breast milk THC concentration (ρ = 0.6 and 0.92, respectively) at both visits (Fig. 1a, b). [...] The cannabinoids THC and CBD are present and accumulate in breast milk after maternal cannabis use, whether by inhalation or edible ingestion. They appear rapidly after exposure and may remain present for at least 1–2 days. Mothers using cannabis during breastfeeding were well educated and used cannabis daily for medical purposes. Their cannabis use increased significantly during the early postpartum period.