Background Previous community surveys of the drop out from mental health treatment have been carried out only in the USA and Canada. Aims To explore mental health treatment drop out in the World Health Organization World Mental Health Surveys. Method Representative face-to-face household surveys were conducted among adults in 24 countries. People who reported mental health treatment in the 12 months before interview (n=8482) were asked about drop out, defined as stopping treatment before the provider wanted. Results Overall, drop out was 31.7\%: 26.3\% in high-income countries, 45.1\% in upper-middle-income countries, and 37.6\% in low/lower-middle-income countries. Drop out from psychiatrists was 21.3\% overall and similar across country income groups (high 20.3\%, upper-middle 23.6\%, low/lower-middle 23.8\%) but the pattern of drop out across other sectors differed by country income group. Drop out was more likely early in treatment, particularly after the second visit. Conclusions Drop out needs to be reduced to ensure effective treatment.