Put simply, LSD does not cause death at recreational or therapeutic doses (less than 500 ug / 0.5 mg). While there are substantial reasons why users should be cautious about LSD use (see LSD Health), death is not a major risk.
Less than a handful of human deaths have been tied in the medical literature to the pharmacological effects of LSD, and none of these deaths have been unquestionably attributable to LSD's actions. The clearest case was documented by Fysh et al. in 1985; however, they fail to explain the circumstances of the death, only discussing the toxicological assessment, casting some doubt that the only explanation for the death was LSD.
Estimates of lethal doses of LSD are higher than 10 mg (10,000 ug) administered orally, more than 100 times a normal moderate dose of LSD (100 ug). The administration of this amount would require the ingestion of more than 200 units of street blotter, which typically contain about 50 ug of LSD (as of late 2010). LSD has been used by tens of millions of people over the last 50 years and has been administered to tens of thousands of patients in psychotherapeutic settings (mostly prior to 1960).
Some suicides have been tied to the use of LSD, though it is difficult to positively link an individual's choice to take their own life with their past use of LSD. In general, LSD is not reported to substantially increase the risk of suicide and those who do commit suicide after taking LSD are likely to have suffered from pre-existing suicidal tendencies.
Some deaths have been associated with inebriated or combative behavior while under the influence of LSD, including falling or jumping from a height or dying after being beaten by police.
Because the numbers of fatalities associated with LSD are so low, it is difficult to determine the risk of death associated with LSD. Erowid estimates that the risk of death from taking LSD is probably less than one death per million LSD use sessions, with risk of death higher among those predisposed to suicide and among those without a sober sitter present to help avoid accidents or fights.